For multi-input multi-output(MIMO) radars, grating lobes appear in the transmitting pattern when the spacing between transmitting antennas and that between receiving antennas are related with some particular values. In this case, high sidelobes occur in the synthetic pattern obtained by multiplying the transmitting pattern with the receiving pattern. This paper analyzes the cause of high sidelobes mathematically, and proposes a novel algorithm that can synthesize a least square error (LSE) receiving pattern with nulls to suppress high sidelobes for MIMO radar. The optimum weighting with nulls is obtained with the Lagrange multiplier method. The LSE receiving pattern is synthesized to solve the high sidelobe problem. Simulation results verify that high sidelobes in the synthesized pattern can effectively be suppressed using the obtained weightings.