One of the popular methods for recognition of facial expressions such as happiness, sadness and surprise is based on deformation of facial features. Motion vectors which show these deformations can be specified by the optical flow. In this method, for detecting emotions, the resulted set of motion vectors are compared with standard deformation template that caused by facial expressions. In this paper, a new method is introduced to compute the quantity of likeness in order to make decision based on the importance of obtained vectors from an optical flow approach. For finding the vectors, one of the efficient optical flow method developed by Gautama and VanHulle[17] is used. The suggested method has been examined over Cohn-Kanade AU-Coded Facial Expression Database, one of the most comprehensive collections of test images available. The experimental results show that our method could correctly recognize the facial expressions in 94% of case studies. The results also show that only a few number of image frames (three frames) are sufficient to detect facial expressions with rate of success of about 83.3%. This is a significant improvement over the available methods., {"references":["D. Darwin, \"The Expression of Emotion in Man and Animals,\" John\nMurray, 1872, reprinted by University of Chicago Press, 1965.","E.W. Young and H.D. Ellis (Eds.), \"Handbook of Research on\nProcessing,\" Elsevier Science Publishers, 1989.","Cohn-Kanade AU-Coded Facial Expression Database. Available:\nhttp://vasc.ri.cmu.edu/idb/html/face/facial_expression/","G.J. Edwards, T.F. Cootes, and C.J. Taylor, \"Face Recognition Using\nActive Appearance Models,\" Proc. European Conf. Computer Vision,\nvol. 2, pp. 581-695, 1998.","H. Hong, H. Neven, and C. von der Malsburg, \"Online Facial Expression\nRecognition Based on Personalized Galleries,\" Proc. Int'l Conf.\nAutomatic Face and Gesture Recognition, pp. 354-359, 1998.","M.J. Black and Y. Yacoob, \"Recognizing Facial Expressions in Image\nSequences Using Local Parameterized Models of Image Motion,\" lnt'l J.\nComputer Vision, vol. 25, no. 1, pp. 23-48, 1997.","J.F. Cohn, A.J. Zlochower, J.J. Lien, and T. Kanade, \"Feature-Point\nTracking by Optical Flow Discriminates Subtle Differences in Facial\nExpression,\" Proc. Int'l Conf. Automatic Face and Gesture Recognition,\npp. 396-401, 1998.","J.N. Bassili, \"Emotion recognition: The role of facial movement and the\nrelative importance of upper and lower areas of face,\" Journal of\nPersonality and Social Psychology, Vol. 37, 2049-2059, 1979.","Y. Yacoob, L. Davis, \"Recognizing human facial expressions from long\nimage sequences using optical flow,\" IEEE Trans. Pattern Anal.\nMachine Intell. 16 (6), 636-642. 1994.\n[10] M. Barlett, P. Viola, T. Sejnowski, L. Larsen, J. Hager, P. Ekman,\n\"Classifying facial action,\" In: Touretzky, D., Mozer, M., Hasselmo, M.\n(Eds.), Advances in Neural Information Processing Systems. MIT Press,\nCambridge, MA. 1996.\n[11] T. Otsuka, J. Ohya, \"Recognizing multiple persons facial expressions\nusing HMM based on automatic extraction of significant frames from\nimage sequence,\" In: Proc. Int. Conf. On Image Processing, pp. 546-\n549. 1997.\n[12] X. Chen, T, Huang, \"Facial expression recognition: A clustering-based\napproach,\" In: Pattern Recognition Letters, Elsevier Science, pp 1295-\n1302, 2003.\n[13] P. Ekman, W. Friesen, \"Unmasking the face,\" Prentice-Hall, 1975.\n[14] Y. Zhang, Q. Ji, \"Facial Expression Understanding in Image Sequences\nUsing Dynamic and Active Visual Information Fusion,\" Proceedings of\nthe Ninth IEEE International Conference on Computer Vision (ICCV\n2003), 2003.\n[15] C. Morimoto, D. Koons, A. Amir, and M. Flicker, \"Framerate pupil\ndetector and gaze tracker,\" In Proc. of the IEEE ICCV99 Frame-Rate\nWorkshop, (Kerkyra, Greece), Sep. 1999.\n[16] ]Z. Zhu, Q. Ji, K. Fujimura, and K. Lee, \"Combining Kalman filtering\nand mean shift for real time eye tracking under active IR illumination,\"\nIn Proc. Int-l Conf. Pattern Recognition, Aug. 2002.\n[17] T. Gautama, M. M. VanHulle, \"A phase-based approach to the\nestimation of the optical flow field using spatial filtering,\" IEEE Trans,\nNeural Networks, VOL. 13(5), September 2002.\n[18] J.L. Barron , D.J. Fleet, and S. Beauchemin, \"Performance of optical\nflow techniques,\" International Journal of Computer Vision, vol. 12(1),\npp. 43-77, 1994.\n[19] D.J. Fleet, and A.D. Jepson, \"Computation of Component Image\nVelocity from Local Phase Information,\" Int. J. Comput. Vision, vol.\n5(1), pp. 77-104, 1990.\n[20] B.D. Lucas, and T. Kanade, \"An Iterative Image Registration Technique\nwith an Application to Stereo Vision,\" 7th International Joint\nConference on Artificial Intelligence (IJCAI), pp. 674-679, 1981."]}