The downlink bit error rate (BER) performances of DS- and MC-CDMA in a frequency-selective fading channel can be significantly improved by the use of frequency-domain equalization (FDE) based on minimum mean square error (MMSE) criterion. However, the uplink BER performance degrades due to strong multi-user interference (MUI). In this paper, we propose frequency-interleaved CDMA using MMSE-FDE, in which the subcarrier components of each user's CDMA signal are interleaved onto wider bandwidth. Then, the frequency-interleaved CDMA signal is transformed into a time-domain signal by the inverse fast Fourier transform (IFFT). Interleaving patterns assigned to different users are orthogonal each other. The proposed scheme can avoid the MUI completely while achieving frequency diversity gain due to MMSE-FDE. It is shown by computer simulation that the use of frequency-interleaving can significantly improve the uplink BER performance in a frequency-selective Rayleigh fading channel.