A wideband MIMO antenna, which consists of two adjacent antenna elements with a very close distance (2 mm or 0.026 lambda), is suggested to achieve a good isolation by the principle of common mode (CM, in-phase signals) and differential mode (DM, out-of-phase signals) cancelation without using any additional decoupling structure in this article. Compared with its reference design, the measured results demonstrate that the isolation of proposed design can be improved to better than 20 dB in a wider bandwidth of 3.2-5.9 GHz (59%), with S-11 and S-22 < -10 dB. The experimental efficiency and envelope correlation coefficients are larger than 78% and <0.04 for the proposed design, respectively. The proposed multiple-input multiple-output (MIMO) antenna array, with good isolation and diversity performance, exhibits broad application prospects in the tightly arranged MIMO antennas of 5G mobile devices.