Radical polymerization of vinyl monomers (styrene, methyl methacrylate, and vinyl acetate) was performed in various nanochannels of porous coordination polymers (PCPs), where relationships between the channel size and polymerization behaviors, such as monomer reactivity, molecular weight, and stereostructure, were studied. The capability for precise size tuning of nanospaces has afforded the first systematic study, of radical polymerization in microporous channels based on PCPs. In this polymerization system, the polymer-growing radicals were remarkably stabilized by efficient suppression of termination reactions in the nanochannels, resulting in living radical polymerizations with relatively narrow molecular weight distributions. A significant nanochannel effect on the polymer stereoregularity was also seen, leading to a clear increase of isotacticity in the resulting polymers.