In this paper, a 2.4 GHz wireless vital signal sensor is proposed to detect heartbeat and respiration signals. The oscillator circuit incorporates with the planar resonator, which functions as a parallel feedback element as well as a near-field radiator. The periodic movement of a body by respiration exercise could cause the input impedance variation of the radiator within near-field range. This variation results in corresponding deviation in the oscillation frequency and it has been utilized for sensing of vital signals. In addition, a SAW filter and power detector have been used to transform the frequency variation into voltage waveform. Also, the relation of the frequency deviation of oscillator and the Q value of resonator is analyzed to increase the system sensitivity. As a result, the experimental results show that the proposed sensor placed 10 mm away from a human body can detect the vital signals very accurately.