We study the performances of a synchronous chip-interleaved, block spread (CIBS) code division multiple access (CDMA) with space-time block-coding (STBC) in the presence of frequency-selective fading. For providing the space diversity gain due to STBC, we introduce the optimum precoding for the STBC. Zero-forcing and minimum mean square error equalizers for CIBS-CDMA are derived. Simulation results confirm that the proposed precoder is valid under the frequency selective fading.