In this letter, a simplified design approach for 2-D multiple-input and multiple-output (MIMO) patch antenna systems is firstly presented and then a new self-decoupling concept is proposed. By equally exciting a pair of degenerate modes in the square patch resonator, i.e., TM_mn and TM_nm, a diagonal distributed status which possesses identical field distributions along all four sides of the resonator is created. In this context, the mutual couplings between horizontal- and vertical-arrayed patch elements become consistent and thus the complex 2-D decoupling problem is effectively facilitated and simplified to a 1-D one. Thereafter, a microstrip line with two metallic pins at both ends connected to the patches is embedded between the patch and ground layers to reduce the mutual coupling. Then, a 1×2 MIMO antenna system with an extremely close edge-to-edge separation of 1mm (0.0111λ₀) demonstrates that the proposed method can realize a remarkable isolation enhancement of 4.3 to 19.8 dB over the entire working band. Moreover, the 1×2 MIMO antenna system is extended to the 2×2 ones, thereby validating the unique feature of the diagonal status of resonant mode in full 2-D MIMO patch antenna system designs.