Respiration-gated radiation therapy is a promising treatment modality that precisely delivers prescribed radiation dose to the lung tumor while minimizing the incidence and severity of normal tissue complications. Conventional gating techniques either rely on implanted fiducial markers or external surrogates such as markers placed on patients' abdomen. They are either invasive to the patients or do not have sufficient accuracy. In this paper, we present a non-contact Doppler radar method to non-invasively measure the respiration signals, from which, accurate gating signals can be derived to control the linac. No marker is needed in our method, which makes it very convenient in use. We measured the respiration using a 5.8 GHz quadrature radar. Analysis of the measured signal is presented. It has been shown that the non-contact means of respiration measurement is able to supply reliable breathing motions and accurate gating signals for radiotherapy of mobile tumors.