A Monte Carlo (MC) simulation study of radical recombination dynamics in solution is presented. Random walks on a three-dimensional lattice are used to simulate the diffusion of radical pairs initially prepared by photolysis. The survival probability as a function of time is calculated by the dynamical interpretation of the MC methods. For the dynamics of radical pairs, the interplay of the geminate and the bulk recombinations is important at high concentrations. In most previous simulations only the dynamics of isolated geminate pairs were considered. In our simulations such interplay is clearly demonstrated by studying the intermediate to long time dynamics of many radical pairs. We compare our results with the predictions of the diffusion and the kinetic theories to examine the validity of their approximations. For low concentrations, results of the simulations show good agreement with theoretical predictions and provide a microscopic basis for understanding the recombination process. But notable deviations are found for high concentrations, which can be attributed to the approximations in theoretical studies.