Abstract: The very low transmission power and the large bandwidth used, enable an UWB system to co-exist with other narrow band systems. The narrow band interference to UWB systems must be considered when DS-UWB systems are designed. In this paper, the method with characteristic function is presented for precisely calculating the bit-error probability of the DS-UWB systems in the presence of CDMA interference in a multi-path environment. In the end, the simulation tests and numerical analysis are carried out based on the IEEE 802. 15.3a channel model. The results show that the characteristic function method is accurate for predicting the BER performance of the UWB systems in the presence of CDMA inteference.