Abstract: Compared with single-carrier direct squence spread spectrum (DSSS), multi-tone (MT)DSSS obtains higher processing gain with equal spectrum efficiency, but it is more sensitive to imperfect carrier synchronization due to spectral overlap between non-orthogonal subcarriers. In this paper, the frequency offset (FO) and phase noise (PN), two patterns of carrier synchronization errors in MT system, are modeled as different random processes respectively. The decision statistics of correlation detection and bit error rate (BER) are derived and calculated theoretically, which are verified through the numerical results. When FO exists, the system performance degrades badly caused by the error of carrier frequency estimation, and degrades slightly due to inter-subcarrier interference (ICI). When PN exists, the ICI is suppressed by low-pass filtering of correlation detector with same spreading code on each subcarrier, while the ICI is not related to PN in the case of different spreading codes on each subcarrier.