Unmanned Aerial Vehicles (UAVs) have been a promising platform in realizing high-speed wireless networks. As an emerging scenario, the UAV communication is distinct from widely used cellular systems or vehicular networks, requiring the development of practical yet easy-to-use channel models. In this paper, for the first time we introduce the geometry-based stochastic model (GBSM) to UAV channel modeling, and propose a new three- dimensional (3D) GBSM for UAV Multi-Input Multi- Output (UAV-MIMO) channels. Based on the proposed model, we derive and investigate the space-time correlation function (STCF) under a 3D moving and scattering environment. The usefulness of this model is verified by the comparison between the theoretical results and some measurement data.