Influences of a Pt underlayer and the preparing conditions on the magnetic and magneto-optical (MO) properties of Co0.25Pt0.75 alloy films, which were prepared by sputtering using a composite target, were investigated. It was found that a good fee (111) texture was formed in a Pt underlayer deposited in a sputtering pressure below 2 Pa even without heating the substrate. Co0.25Pt0.75 alloy films subsequently deposited on such a Pt underlayer exhibited an enhanced perpendicular magnetic anisotropy, and the growth temperature for excellent magnetic and MO properties was lowered to 150 degreesC. Static read-write experiments on the Co0.25Pt0.75 alloy films showed that the static signals could he easily written, at. a moderate recording power, in the alloy films without a Pt underlayer, and that the introduction of a relatively thick Pt underlayer was not beneficial to the thermomagnetic writing for the alloy films. The large readout signals of an Al/ZnS/Co0.25Pt0.75/glass trilayer structure suggest that CoxPt1-x alloy films are a promising candidate for the advanced MO recording media.