A method of preparing a supported catalyst, a supported catalyst prepared by the method, and a fuel cell using the supported catalyst. In particular, a method of preparing a supported catalyst by preparing a primary supported catalyst containing catalytic metal particles that are obtained by a primary gas phase reduction reaction of a portion of the final loading amount of a catalytic metal, and reducing the remaining portion of the catalytic metal by a secondary liquid phase reduction reaction using the primary supported catalyst. The supported catalyst contains catalytic metal particles having a very small average particle size, which are uniformly distributed on a carbon support at a high concentration, and thus exhibits maximal catalyst activity. A fuel cell produced using the supported catalyst has improved efficiency.