The influence of transiently bound collision on the diffusion coefficient of free radicals was examined using molecular dynamics simulations and the Green-Kubo formula. It was found that transiently bound collisions significantly increase the diffusion coefficients of free radicals at temperatures relevant to combustion. The present study suggests that a molecular theory beyond the Chapman-Enskog equation is needed to evaluate the diffusion coefficients of free radicals in laminar flame and other high-temperature reacting flow simulations.