Addition reaction of a radical to the double bond of a monomer, which is important at early stage of photopolymerization, has been studied by time-resolved (TR-) and pulsed electron paramagnetic resonance (EPR) spectroscopic methods. Reactions, of One phosphorus-centered and three carbon-centered radicals attacking to several monomers have been employed. Intermediate radicals were identified by analyzing the I recorded TR-EPR spectra, and the reaction rate constants were determined by the electron spin-echo detection method proposed by Weber and Turro [J. Phys. Chem. A, 2003, 107 (18), 3326-3334]. The quantum chemical calculation shows that the rate constants for the addition reactions are well-explained by introducing two factors of "enthalpy effect" and "polar effect" to control the activation barrier height. It was observed that the rate,constants of the phosphorus-centered radical were larger than those of carbon-centered radicals for some monomers. The difference in the rate constants was argued on the basis of frequency factor and activation barrier both of which are influenced by an atom of radical center.