The adaptive modulation based on adaptive-interval and mode-switching for OFDM systems in time-varying channels is proposed. The performance of mode-switching-assisted adaptive modulation is closely dependent on the mode-switching levels. The conventional scheme use fixed mode-switching levels that is derived in AWGN channel or variable mode-switching levels according to the average SNR. But the conventional schemes don’t consider the effect of adaptive modulation interval and user mobility. Due to user mobility, the BER performance is degraded in a long interval. If short interval is used, the feedback information and overhead is increased. The proposed scheme derived the adaptive modulation interval and corresponding mode-switching levels that maximize the spectral efficiency.