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Based on Basel II Accord, loans paid to individuals and SMEs are included in retail portfolio and banks are permitted to choose standardized approach or internal rating based approach for calculating their credit risk capital requirements. In the case of IRB Implementation, banks should group their retail loans into homogenous risk pools. Particularly, IRB capital requirement function is related to probability of default (PD) and Loss given default (LGD) for each borrower. Mathematically, capital requirement function is concave in PD for a given LGD and for a widespread interval. As a result of capital requirement function concavity, banks could lower their overall capital requirement through classification of their loans into more homogenous risk pools. In this study, loans paid to individual retail customers of 1343 for one of the private banks during 1391-1392 have been classified into homogenous risk pools by the Classification and Regression Trees (CART) algorithm. As we go from level 0 to level 5 in customers' segmentation scheme, capital required for bank experiences a decline of 0.44%.