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Purpose: The radiation injury of the rectum and urethra is the major concern in radiotherapy for prostate cancer because it often threatens a patient's QOL. There are many references available for the dose constraints in conventional radiation therapy, however, no clear index has been established for carbon ion therapy. The aim of this study is to fit the normal tissue complication probability (NTCP) model to clinical outcome on rectal and urethral late complication after carbon ion therapy for prostate cancer. \nMethod: Rectal and urethral dose-volume histograms (DVHs) and clinical records of 180 prostate cancer patients were considered. These patients were treated with 54-72 Gy (RBE) including dose escalation trial at the Research Center for Charged Particle Therapy. The end point for analysis was Grade 2 or worse complication probabilities. All rectal and urethral late complications were graded using a modified scale and criteria from the Radiation Therapy Oncology Group (RTOG) and the European Organization for Research and Treatment of Cancer (EORTC). NTCP models were considered the Lyman-Kutcher-Burman (LKB) model with DVH reduced to the equivalent uniform dose (EUD). The parameters for the model were fit to the patient data using the maximum likelihood analysis. \nResults: Of the 180 patients, 12 and 26 experienced Grade 2 or worse late complications for the rectum and urethra after carbon ion therapy. The parameters obtained from the fit of the LKB model were n for the volume effect, TD50 for the dose associated with 50% chance of complication after uniform whole irradiation and m for the steepness of the dose-response curve. The values of n, m and TD50 offer a good prescription of the prostate distribution in our population. Compared to the use of this model with previously published parameters for conventional radiation therapy, the rectal NTCP parameter n and m were smaller value for carbon ion therapy. This means a rectum behaves more serial organ-like for carbon ion therapy and have wider range of uniform doses. \nConclusions: The NTCP model fit quite accurately the considered clinical data for the rectum and urethra. It seems that there is a difference of the rectal NTCP parameters between carbon ion therapy and conventional radiation therapy. A new NTCP parameter values are useful to compare the plans for carbon ion therapy., ESTRO33