This paper presents the design of Fermi antenna array with improved gain for radar respiration measurement. A six-element Fermi antenna array working at 5.8 GHz is optimally designed and integrated with the respiration radar system. Specifically, to easily feed the antenna array as well as reduce the total size of the array system, a feeding network based on 0-degree phase shifters is employed along with suitable impedance transformers. The whole system is proposed to function as a radar sensor for respiration rate measurement, which is known as a promising non-contact and non-invasive approach for lung cancer radiotherapy. The measurement results verify the improved performance of the proposed system for radar respiration measurement in motion-adaptive lung cancer radiotherapy.