An exact form of symbol error rate (SER) is derived for zero-forcing decision feedback equalization (ZF-DFE) on fast fading channels. We present an error propagation framework in ZF-DFE, which is dependent on the length of multipath and the size of M-ary quadrature amplitude modulation (M-QAM). By applying the concepts of “error event” and “state” on this framework, we can evaluate the error probability considering the error propagation. Compared with the previously obtained results, our approach gives an accurate SER expression for Gray-mapped M-QAM on L-tap multipath fading channels. The simulation results are provided to verify the accuracy of the new error rate expression.