In this paper, a novel compact T-shape defected ground structure (DGS) is proposed in order to reduce the mutual coupling between elements in a microstrip antenna array structure. The proposed DGS has considerable advantages compared to an H-shaped DGS. As is known, DGS insertion is a common problem in microstrip antenna array structures. However, the proposed T-shaped structure is more compact so it could be suitable for a compact microstrip array antenna. The T-shaped DGS is about 24% more compact than the H-shaped one. However, the most important novelty of our work is reducing mutual coupling both in the E-plane and the H-plane, which previously was discussed just in the E-plane. Finally, we have considered the disturbing effect of mutual coupling on the side lobe level (SLL) of unequally spaced arrays; using these results, a T-shaped DGS has been implemented in an 8-element unequally spaced array antenna in order to reduce its SLL.