This study presents a periodic defected ground structure (PDGS) to suppress the mutual coupling (MC) between coplanar antenna elements in a multiple‐input and multiple‐output (MIMO) communication system. The proposed PDGS consists of three defected ground structure units. They are etched away from the ground plane between antenna elements. The coupled polarisation transformer characteristic of the proposed PDGS is utilised to increase the antenna elements isolation. The increased coupled vertical linear polarisation component and decreased coupled horizontal linear polarisation component contribute to reduce the MC between antenna elements. By using this method, the authors achieve low MC level and better MIMO corresponding capacity. This method has little influence on the far‐filed main radiation properties of the antenna elements.