Ni epitaxial thin films were prepared on Au(100)_fcc underlayers hetero-epitaxially grown on MgO(100)_B1 single-crystal substrates by ultra-high vacuum molecular beam epitaxy. The effects of substrate temperature on the structural and the magnetic properties were investigated. hcp-Ni crystals are successfully obtained at temperatures lower than 300°C. With increasing the substrate temperature, the volume ratio of more stable fcc phase increases in the film. The hcp-Ni crystal consists of two types of (11-20) domains whose c-axes are rotated around the film normal by 90°each other. X-ray diffraction shows that the Ni film prepared at 100°C consists primarily of hcp crystal. An atomically sharp boundary is recognized at the Ni(11-20)_hcp/Au(100)_fcc interface for the Ni film. The in-plane magnetization properties of hcp-Ni film are interpreted to be reflecting the magnetocrystalline anisotropy of bulk hcp-Ni crystal which was predicted by a computer simulation study.