While higher chip rate can provide better performance for Direct Sequence/Code Division Multiple Access (DS/CDMA) systems due to larger process gain, it may also induce spectrum emission to adjacent channels, i. e. , adjacent channel interference. Especially, if different operators use adjacent channels in the same area with uncoordinated power levels, such interference becomes large, and excessively higher chip rate will decrease the efficiency of a system. In this context, this paper evaluates the relation between chip rate and capacity in DS/CDMA cellular communication systems considering adjacent channel interference from other systems. First, the classification of adjacent channel interference between two independent DS/CDMA systems is described, and the concrete interference levels are calculated for several chip rates. Then, by using computer simulation, the system CDMA capacity is evaluated under adjacent channel interference. From these results, we can find that the excessively higher chip rate can not always provide the larger system CDMA capacity in spite of the larger process gain, and there exists the appropriate chip rate for a certain given bandwidth.