The most frequently used methods for distortion measurement are fast Fourier transform (FFT) in frequency domain and curve fitting in time domain. However, FFTbased method needs integral period synchronous sampling, in order to avoid picketfence effect and spectrum leakage. On the other hand, curvefittingbased method has higher computational burden, and it is difficult to ensure the robustness. To overcome the above shortages of FFTbased and curvefittingbased approaches, discretetime Fourier transform (DTFT) can be used to extract and eliminate the fundamental wave from the measured signal, and then the total harmonic distortion (THD) can be calculated. Neither curve fitting nor FFT is needed in the proposed method, thus the computation is simplified. Numerical simulation and experiment for the proposed method are performed with 12 bit sampling resolution. Simulation results of sinusoidal signal with low distortion show that the absolute error of THD is less than 0.05%, while the experiment results of 20 Hz~20 kHz square and triangle waves have less than 1% absolute error. Therefore, the proposed method is efficient and reliable for distortion measurement.