Polymer electrolyte fuel cells suffer from reduced lifetimes due to degradation of their Pt catalysts during operation. To understand the fundamental process of the Pt degradation, we proposed a model for the Pt particle growth based on the Gibbs-Thomson equation, which asserts that smaller particles tend to be dissolved in preference to the larger ones. We simulated the particle distribution changes during rectangular potential cycling between 0.6 and 1.0 V vs. the reversible hydrogen electrode at 25 °C under a N2 atmosphere. The parameters in our model were determined by fitting to the experimental data. The calculation results and experimental data for the changes in the particle distribution and electrochemically active surface area were in good agreement. Additionally, the particle distribution change under different conditions such as the potential range and the initial particle size distribution could be simulated by changing the parameters in the model. When the initial size standard deviation is low, particle growth does not readily occur because the differences in the particle size are small. When the initial standard deviation in the particle size is large, the particle growth is accelerated by the large difference in the particle sizes, because small particles more readily dissolve. Finally, the particle distribution becomes stable and the degradation levels off. It was suggested that the particle growth could be anticipated by using our model.