The simulation of free radical crosslinking polymerizations of multifunctional monomer by a percolation model was performed on a two-dimensional lattice with periodical boundaries. The model was formulated in such a way that a variety of monomers, translational diffusion of monomer and polymer molecules, segmental diffusion of polymer, shielding effects, microgel formation, and unequal reactivities of vinylene bonds can be considered. Simulation results of the 2-D model agreed well with those of a 3-D model. The percolation model was able to qualitatively verify several experimental results.