Microwave/millimeter-wave Doppler sensor systems with various operating frequency for noncontact human vital-signs sensing have been reported extensively. 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 sensor and calculate the radar cross section (RCS) of different frequency. A pig heart is used in the experimental measurement to measure the RCS and compare with the computed value of the spherical muscle sphere at 60 GHz. The receiving power of the Doppler sensor can then be calculated from the radar equation at different frequency. 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 sensor 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. Measured vital signs of a 60-GHz Doppler sensor system of a human subject 2-m away are also demonstrated.