This paper analyzed the performances of multiuser detector introduced in MC CDMA system. The multiuser detector scheme is decorrelating parallel interference canceling. It is compared with Hara's single user detector in the same slow fading Rayleigh multipath channel.Both uplink and downlink channels are analyzed. The simulation results show that if there is no channel estimation error, in the downlink, single user MMSEC detector is better, while in the uplink, the performance of the multiuser detector is far better. Further analyzing the effect of channel estimation error on the detectors, it is found that the multiuser detector is more sensitive to big estimation error than the single user detector. The conclusion is that single user MMSEC detector is only suitable for the downlink MC CDMA system, while the multiuser detector is a wonderful scheme for the uplink MC CDMA system at small channel estimation error.