An improved method of making a magnetic recording medium with excellent coercive force and rectangular ratio is provided by means of an ion plating technique. A support and an evaporation source of a magnetic Co-Si alloy are placed in an ion plating apparatus which is filled with an inert gas and kept at low vacuum. High voltage is then applied between the support and the evaporation source such that the former has a negative polarity and the latter a positive polarity, thereby causing a glow discharge zone in the apparatus. The magnetic alloy is evaporated into the glow discharge zone from the source to form a magnetic thin film on the support. Such a magnetic thin film can have a coercive force of 750 Oe or more and a rectangular ratio of 0.80 or more and can be used as a magnetic recording medium.