The mechanism of cyclopolymerization was investigated by using N-n-propyldimethacrylamide (PDMA). Completely cyclized polymers were formed on polymerization of PDMA by a radical initiator. Moreover, those copolymers of PDMA and various monomers, such as styrene, methyl methacrylate, and vinyl acetate, obtained did not contain any detectable pendent double bonds. The kinetic investigation showed that the termination reaction proceeded between the cyclized radicals. The attempted polymerization of N-n-propyl-N-isobutyrylmethacrylamide, the monofunctional counterpart of PDMA, was failed. These results appear to confirm that cyclopolymerization of PDMA proceeds through a concerted mechanism which has been proposed for the mechanism of the cyclopolymerization of various difunctional monomers. Measurement of the ESR spectra of propagating radical has, however, revealed that the rate-determining step of the cyclopolymerization of PDMA is not intermolecular propagation but intramolecular cyclization, which indicates that the cyclization reaction proceeds in a stepwise way. This apparent contradiction was explained based upon thermodynamic considerations.