CoNi epitaxial thin films were prepared on MgO(100) and SrTiO3(100) single-crystal substrates by ultra-high vacuum molecular beam epitaxy. Effects of substrate temperature on the structural and magnetic properties of epitaxial films were investigated. On MgO(100) substrate, CoNi epitaxial films consisting of hcp(112Â¿0) and fcc(100) crystals are obtained whereas on SrTiO3(100) substrate, CoNi(100) single-crystal films with fcc structure are formed. CoNi film growth on both substrates seems to follow the Volmer-Weber mode. CoNi (100)fcc single-crystal film with smaller strain is obtained on SrTiO3 substrate rather than on MgO(100) substrate. The film strain decreases with increasing the substrate temperature. The magnetic hysteresis loops of CoNi epitaxial thin films reflect the magnetocrystalline anisotropies of hcp-CoNi and fcc-CoNi crystals.