The kinetics and chain length distributions occurring in living free-radical polymerizations are simulated using a hybrid Monte Carlo algorithm. The new algorithm is much faster than the conventional one because the activation/deactivation exchange reactions, which are CPU intensive, are treated by a biased-sampling method with an analytical expression for the exchange equilibrium, while the reactions of chain propagation, irreversible chain termination, etc. are treated by exact stochastic Monte Carlo simulation, Two models of living radical polymerizations, i.e., the polymerization initiated by alkoxyamines and the nitroxide radical, 2,2,6,6-tetramethyl-1-piperidinyloxy, mediated radical polymerization, are simulated to study the effects of experimental variables, such as the concentration ratio of stable free radicals to initiators, initiation rate constants, etc., on the kinetics and molecular weight distributions. A comparison between simulated and analytical results in the literature is made. Taking thermal. initiation into consideration, the algorithm reproduces the experimental results very well. Therefore, its feasibility and usefulness in studying living free-radical polymerization are demonstrated.