Abstract-A loss of large aperture quasi-optics which consist of a lens and a feed antenna is firstly measured using a radiometer receiver via the modified reference control method for a W-band imaging radiometer system. The quasi-optical loss is mainly decided by the dielectric loss of the lens with good quasi-optical transformation efficiency between the lens and the feed antenna. The quasi-optics composed of an aspheric lens and a dielectric rod antenna are designed for high resolution, low aberration, and compact size. The fabricated quasi-optics with the aperture diameter of 500 mm have the quasioptical transformation efficiency of more than 95%. The radiometer receiver is designed applying a total power type and a direct conversion topology for simplicity, compact size and low temperature sensitivity. The manufactured receiver has the temperature sensitivity less than 1 K for both a hot source and a cold source. The calculated and measured results of the quasi-optics are very well matched by approximately 1.6 dB. The expected measurement errors by the reference control method are also analyzed as the functions of the characteristic parameters of the radiometer receiver.