Lung cancer radiotherapy is subject to tumor motion caused mainly by respiration, due to the internal organ link. Motion-adaptive radiotherapy delivers most effective radiation dose to the tumor, while minimizes the damage to normal tissues. Accurate tumor tracking, an attractive motion-adaptive radiotherapy, requires accurate measurement of respiration at multiple body locations. Radar respiration measurement is a promising noncontact and noninvasive approach for lung cancer radiotherapy. However, the limited room for integrating radar sensors in the linear accelerator calls for miniature but high-performance antennas. In this paper, compact antenna array is employed for radar respiration measurement in motion-adaptive lung cancer radiotherapy. Potentially, one radar is enough to simultaneously measure respiration at chest and abdomen. Simulation and experimental results show that the proposed antenna array serves as a good option for radar respiration measurement in motion-adaptive radiotherapy.