A W-band low sidelobe level offset-fed reflectarray antenna is designed, fabricated, and measured. Compared to conventional offset-fed reflectarray antenna, the sidelobe level of proposed one is decreased significantly when the inclination angle of reflector is half of the incident angle of the feeding. At the same time, a large-radiation-area element is used to obtain low sidelobe level for the offset-fed reflectarray antenna because of its large radiating element area and low specular reflection. A 52 mm×180 mm offset-fed reflector antenna have been designed and measured to verify the availability. From 90 to 96 GHz, measured results show that a maximum gain of 36dBi at 93 GHz, and the peak sidelobe level of 18dBc can be obtained with the proposed architecture.