This letter analyzes the modulation sensitivity of Doppler radar vibration measurement using both 24 GHz and 2.4 GHz Doppler radars. A rigorous analysis is developed in terms of radar modulation sensitivity. In contrast to the common observation that the modulation sensitivity increases as the carrier frequency increases, it is shown that, for a given vibration motion, there exists a specific carrier frequency for optimal measurement. Experimental demonstration was carried out using both 24 GHz and 2.4 GHz radars to measure the same vibration, and the signal-to-noise ratio (SNR) at the receiver input was compared. It is shown that for cm scale movement, 2.4 GHz radar has a higher modulation sensitivity, while the 24 GHz radar has the optimal modulation sensitivity in the mm scale.