The partial parallel interference cancellation (PIC) detector is considered as an effective yet simple receiver for multiuser detection in code-division multiple access (CDMA) systems. It cancels the multiple access interference (MAI) of the received signal partially resulting from other users. However, the optimal cancellation level which is controlled by a partial cancellation factor (PCF) is usually determined by trial and error, since no theoretical values have been reported. We introduce a MMSE criterion for the two-stage partial PIC receiver to derive the optimal PCF. Simulations show that the performance of proposed methods is very close to that of empirically chosen PCF. We also demonstrate that the derived PCF is insensitive to the estimate bias of noise level and the interfering signal power.