Objective To establish an effective method for evaluating muscular fatigue by using surface electromyography (sEMG) signals under electrical stimulation or rest condition. Methods Both sEMG signals under electrical stimulation conditions and rest condition were recorded during the experiments of muscular fatigue indicated by motion contraction and electrical stimulation. Power spectrum by Fourier transform and approximation entropy were extracted to analyze the signals. Results There were left shifts found on the curves of power spectrum in both voluntary and evoked sEMG signals on occurrence of fatigue，whereas no obvious shift on the curves of the resting ones. In the entropy analysis，there was a monotonous decrease in voluntary signal and an up and down tendency in electrical stimulation signal. Conclusions Both methods are good measures to indicate muscular fatigue，and are more indicative in active (i.e. voluntary and electrical stimulation) conditions than in rest conditions. And approximation entropy is more suitable to indicate fatigue of sEMG signals evoked by electrical stimulation.