In this paper, a Left-handed Metamaterial (LHM) structure is designed for side lobe suppression of a microstrip array antenna at frequency 2.8-3.1 GHz. The LHM structure is placed at the top of the microstrip array antenna with a space or gap between them. Side lobe suppression is very important for Radar system which needs antenna with high performance of its radiation properties. The linier array microstrip antenna consists of 4x1 elements. The simulation result using CST microwave studio shows that the Side Lobe Level (SLL) has been suppressed from -8.93 dB to -15.86 dB at phi = 0 while the measurement result shows suppression from -6.6 dB to -10.75 dB. Both simulation and measurement result shows a side lobe suppression using LHM structure.