A simulation model for arsenic diffusion in polycrystalline silicon has been developed considering dynamic dopant clustering and polysilicon grain growth kinetics tightly coupled with dopant diffusion and segregation. It was assumed that the polysilicon layer consists of column-like grains surrounded by thin grain-boundaries, so that one dimensional description is permissible for dopant diffusion. The dynamic clustering model was introduced for describing arsenic activation in polysilicon grains, considering the solubility limit increase for arsenic in a polysilicon. For a grain-growth calculation, a previous formula was modified to include a local concentration dependence. The simulation results show that these effects are significant for a high dose implantation case.