Electron Beam Cured (EBC) coating of epoxy-acrylate resin as oligomer has been used in many areas. The microstructure of the coating is closely correlated with the molecular structure of the components and radiation dose during curing. The curves of S parameter vs. incident energy of the positrons were measured to study the polymerization and crosslinking induced during electron irradiation curing. The bulk S-parameter (S-b) and the effective diffusion length (L-eff) were used to characterize the epoxy acrylate coating. The correlations of S-b and L-eff with the content and molecular structure of reactive diluents, and irradiation dose are discussed.