The thermal decomposition of a radical initiator, AIBN in a microreactor was examined. The plots of unchanged AIBN against the residence time fitted well to the calculated one based on the bath temperature, indicating the efficient heat transfer through the wall of the microreactor by virtue of a high surface-to-volume ratio. The effectiveness of the microreactor on the molecular weight distribution control was then examined. For the polymerization of butyl acrylate (BA), the polydispersity index (PDI) of the polymer obtained using the microreactor was much smaller than that obtained with a macroscale batch reactor. The result can be explained in terms of much higher heat removal efficiency of the microreactor compared with the macroscale batch reactor. For the polymerization of benzyl methacrylate (BMA) and methyl methacrylate (MMA), the effect of the microreactor on PDI was smaller than in the case of BA. For the polymerization of vinyl benzoate (VBz) and styrene (St), no appreciable effect of microreactor on PDI was observed. These experimental results indicate that the microreactor is quite effective to the molecular weight distribution control for highly exothermic free radical polymerizations (BA, BMA, and MMA) but that it is not so effective for less exothermic polymerizations (VBz and St).