A simulation model for the formation and evolution of frazil in open channels is developed. In this model, the primary nucleation is assumed to be due to mass exchange of seeding crystals at the free surface. The model of frazil crystals growth is based on the rate of heat transfer between crystals and the ambient turbulent flow. Secondary nucleation and flocculation are simulated based on binary collisions of frazil particles. The model is validated with existing experimental data. It is capable of simulating the variation of water temperature during the frazil formation period and the evolution of size and concentration distributions of frazil in the flow. Effects of the surface heat exchange, the rate of seeding, and the flow condition are examined