Abstract: Multiple-input multiple-output（MIMO） technology is the development trend of modern communication technology, which provides great support for the rapid mobile changes. In this paper, the isolation of a novel ultra wideband（UWB） MIMO antenna is studied and improved. The-10 dB impedance bandwidth is expended from 3.3 GHz to 9.1 GHz. Through cutting a wide slot on the antenna ground plate and then adding a fork-branch on the ground, the isolation between the ports of the ultra-wideband MIMO antenna can be obviously improved. The simulated results show that the impedance bandwidth is from 2.9 GHz to 9.8 GHz and the isolation is more than-19.3 dB in the operating band with the defected ground structure（DGS）.