The transmit array of phased MIMO radar is partitioned into a number of fullyoverlapped subarrays. Considering the performance of sidelobe, to determine the optimal subarray number, beforming of phased MIMO radar is studied. The beampattern is expressed as the product of transmit beampattern, waveform diversity beampattern and receive beampattern. Through designing the position of sidelobe and zero point, this paper proposes a method to estimate the optimal subarray number. If the relationship of subarray number, transmit and receive array elements in this paper is satisfied, the best performance of sidelobe can be achieved.