A millimeter-wave wideband antenna is presented for the 5th generation applications. The operation band ranges from 24 GHz to 39 GHz which covers most of the Ka band. Furthermore, a multiple-input-multiple-output (MIMO) antenna is developed. The high isolation is achieved without introducing external decoupling structures. The transmission coefficient is under −20 dB within only 0.4 mm space between antenna elements. The radiation pattern also shows the stability within the wide operation band. Both simulated and measured results show that this proposed MIMO antenna is suitable for the future wireless communications.