In this work The factors affecting the rate of addition of methyl radical to a number of monosubstitued Alkenes have been studied using quantum mechanical calculations. These factors include physical properties such as: electron densities (Total ED)on the carbon atoms involved in the addition reaction (carbon bearing the single electron in methyl radical and the carbon on the unsubstituted methylene carbon in Alkene),electron density on the substituted carbon, electron density on HOMO orbitals, energy level of HOMO orbital (EHOMO) and energy level of LUMO orbital (ELUMO). In addition, the steric energy in the transition state of the addition reaction also has been considered. The correlation between the logarithm of the addition rate constant (logK) and the physical properties has been established. The results of the regression analysis gave the best correlation coefficient (R=0.97) in the case of using (log K) as a dependent variable with three an independent variables which were: polarity, EHOMO, ELUMO. The most influencing factor on the rate of addition was the polarity followed by ELUMO while EHOMO and steric energy showed an insignificant effect.