A novel diffraction lens quasi-optical monopulse antenna/feed is proposed and analyzed using finite-difference time-domain (FDTD) method. The structure consists of diffraction lenses, a quasi-optical sum- difference network, matching lenses and horns connecting waveguide. This antenna structure is novel, low-loss and compact and may be designed to be polarization-independent. Owing to its low absorptive loss and relative easy for manufacturing, this monopulse antenna/feed structure is suitable for millimeter wave and submillimeter wave application. The parameters of the structure are selected to some extent to get better performance. Calculation results are presented to show its properties.