In this paper, a digital sum-difference monopulse system based on four-subarray microstrip antenna is presented. In transmitting, a high gain transmitting beam is obtained from spatial power combining. Gain of transmitting beam is better than 29.5dB. In receiving, we remove sum and difference network. Sum and difference signals are obtained by addition and subtraction operations of signals received from microstrip antenna with four subarrays. Noise figure of four-channel receiver is ameliorated by removing millimeter wave sum-difference network. Phase balances between the four transmitting channels are adjusted by dielectric loaded method[l]. Amplitude and phase imbalances between receiving channels are compensated in signal processing module.