This paper presents a mathematical model of platinum catalyst degradation. Equation describing the degradation of electrochemical surface area on the base of mechanisms: platinum nanoparticles electrochemical dissolution, particle growth due to Ostwald ripening, migration of nanoparticles along the carbon support, coalescence of fine particles, diffusion of platinum ions in the ionomer and their escape into the membrane has been obtained for the first time. Calculations has been carried out for platinum catalysts of two types: commercial and synthesized over carbon nanotubes. As a result of this simulation the evolutions of the platinum particle size distribution vs time have been obtained, values of electrochemical surface area vs time have been calculated and compared to available experimental data. The model allows to predict the operating life of a fuel cell.