A scheme for radar cross section (RCS) reduction of microstrip antenna array in wideband using artificial magnetic conductors (AMC), without compromising the radiation characteristics of the antenna array, is proposed. This design is based on the principle of passive cancellation. The novelty is that the reflection characteristics of the microstrip antenna array are also taken into consideration during the design process of AMCs. The aperiodic configuration is composed of three kinds of AMC lattices with selected dimensions and is applied to the design of microstrip antenna array for the purpose of RCS reduction. The simulated results show that the monostatic RCS is reduced over a wideband from 15.2 to 35 GHz (about 79% relative bandwidth), covering the operation band (20-20.75 GHz) of the antenna array. In addition, compared with the periodic configuration, it has about 4 dB lower maximum bistatic RCS.