The kinetics of chain-length-dependent bimolecular terminations in free-radical polymerization during unstationary state periods are investigated by employing Monte Carlo simulations in segregated microreactors where pseudobulk kinetics is valid. We investigate two types of unstationary conditions : (1) the pre-stationary-state period, in which the radical concentration increases from zero to the stationary-state value, and (2) the posteffect period, in which the radical concentration decreases from the stationary-state value toward zero. The simulations are made to investigate the time developments of radical concentrations and the molecular weights, and the results are compared with those for the cases where a constant termination rate coefficient that is equal to the number-average termination rate coefficient at stationary state is used. The transient periods are longer than those for the cases with constant termination rate coefficients, and the differences become larger as the chain length dependence is enhanced. On the other hand, however, the chain transfer reactions may dominate the stoppage of growing polymer radicals at the intermediate to high conversion regions due to a significant decrease in the bimolecular termination rate coefficient. In such cases, the differences from a constant bimolecular termination rate coefficient become negligible.