A new Monte Carlo simulation method for chain-length-dependent kinetics is proposed. This method employs the segregated microreactor in which pseudobulk kinetics is valid, and the formation process of all polymer molecules involved in the microreactor is simulated by application of the competition technique where only a small number of random numbers are required to generate each polymer molecule stochastically. The necessary volume of the microreactor to simulate the molecular weight distribution (MWD) in homogeneous polymerizations with sufficient accuracy is so small that it may contain about 5 radicals; therefore, the amount of calculation required is rather small. For intermediate to high conversions, bimolecular terminations are likely to be dominated by interactions between radicals with short and long chain lengths. When bimolecular termination by disproportionation is significant, short oligomeric chains that are large in number but small in weight are formed. If bimolecular termination is solely by combination, the instantaneous MWD formed in these conversions may be approximated well with the most probable distribution because all macroradicals are stopped by small molecules including oligomeric radicals via bimolecular termination and monomer molecules as a chain transfer agent. On the other hand, the MWDs formed at low to intermediate conversions are more complex since the probability of chain stoppage for polymer radicals changes rather gradually with chain length. It is straightforward to extend the present simulation to the unsteady state, and it was found that the molecular weight development during the start up period can be significantly different from the predictions based on the assumption of a single constant termination rate coefficient.