SmCo5(0001) epitaxial films were prepared on Cu(111) single-crystal underlayers formed on Al2O3(0001) substrates at 500 °C. The nucleation and growth mechanism of (0001)-oriented SmCo5 crystal on Cu(111) underlayer is investigated and a method to control the nucleation is proposed. The SmCo5 epitaxial thin film formed directly on Cu underlayer consists of two types of domains whose orientations are rotated around the film normal by 30° each other. By introducing a thin Co seed layer on the Cu underlayer, a SmCo5(0001) single-crystal thin film is successfully obtained. Nucleation of SmCo5 crystal on Cu underlayer seems controllable by varying the interaction between the Cu underlayer and the SmCo5 layer.