Abstract: The development of a Ka-band monopulse variable polarization antenna is presented, based on polarization synthesis theory. This kind of microstrip array antenna consists of several linearly polarized subarrays of rotational symmetry, and change the polarization mode by adjusting the phase of each subarray. The circular polarization of three subarrays and four subarrays has high polarization purity with over 30dB cross-polarization and less than 3dB axial ratio within the half power beamwidth; and the imbalance of two peaks in difference beam is less than 1dB, and null depth is over 30dB.