Microstrip antenna array mounted over electromagnetic bandgap (EBG) substrate was studied The EBG was mounted below the microstrip antenna array to replace the conducting ground plane of antenna array, which can widen the operational bandwidth and promote the gain of antenna array. The measured results show that the operational bandwidth of antenna unit is promoted from 3.81% to 8.86%, and in the operational band the gain of antenna array is over 10dB. Meanwhile the coupling coefficient between units of antenna array is reduced.