Highly ordered MnPt3 alloy film was grown by molecular-beam epitaxy (MBE) at a temperature of 300 °C. The MnPt3 film exhibits a large Kerr rotation, which reaches a maximum value of −0.7° at 1.24 eV. MnPt3(2 nm)/Co(0.6 nm) multilayers were also grown by MBE method onto MgO(111) substrates at a temperature of 300 °C. It was verified by the observation of the reflection high-energy electron diffraction patterns that Co and MnPt3 layers in the film were grown epitaxially on MgO(111). The appearance of superlattice line due to Cu3Au structure in x-ray-diffraction patterns confirmed the formation of the ordered phase in the MnPt3 layer. The hysteresis loops for in-plane and film normal directions became close to each other, which implies that the perpendicular magnetic anisotropy was induced through the multilayered structure. The Kerr spectrum of the MnPt3/Co multilayer have a negative peak of −0.26° around 1.4 eV. This value is fairly smaller in magnitude compared to the one estimated by the effective reflectivity method using the optical and the magneto-optical constants for Co and MnPt3 alloy.