The Doppler radar for noncontact human vital-signs sensing have been reported extensively. In order to investigate the frequency effect on the detection performance of the vital signs, the muscle sphere is used to emulate the human heart organ to compute the scattering field of the incident wave radiated from the Doppler radar. The radar cross section (RCS) can then be calculated from the receiving power by radar equation. In the validated experimental measurement, a pig heart (to simulate the human heart) is used to measure the receiving power. RCS and compare with the computed value at 60 GHz. The receiving power of the Doppler radar can then be calculated from the radar equation at different frequencies. From the theoretical upper bound value of the antenna gain (under the fixed antenna size), by observing the simulated receiving-power comparison, it is found that the Doppler radar can receive more reflected power (from the spherical muscle sphere) for the higher carrier frequency (up to 60 GHz) which may have a better vital-signs sensing performance.