The dynamic analyses of FM discriminators-Slope, Travis, Foster-Seely, and Ratio types-are presented by the method of Fourier Transform. The relationships between the distortion of the output waveform from the discriminator and its input FM wave and circuit constants are clearly shown in this study. According to the results, the Travis, Foster-Seely, and the ratio discriminators contain only odd harmonics, but the slope discriminator contains both even and odd harmonics. The permissible handwidth of travis discriminator for the 2% distortion is about twice those of Foster-Seely and ratio discriminatosr and sixteen times that of slope discriminator.