Disclosed is a method for preparing a platinum/support catalyst or a platinum alloy/support catalyst, including: a) preparing a dispersion solution including urea, a support and a water-soluble salt of at least one metal(s) having catalytic activity; (b) reacting the dispersion solution at high temperature so as to deposit the metal hydroxide particles derived from the at least one metal(s) on the support; and (c) reducing the metal hydroxide particles. The size and distribution of the platinum particles or platinum alloy particles are greatly improved by the use of urea.