Well-resolved electron spin resonance (ESR) spectra of the propagating radicals in radical polymerizations of conjugated dienes (1,3-butadiene, 1,3-pentadiene, 2-methyl-1,3-pentadiene, 1,3-hexadiene, 2,4-hexadiene, isoprene) have been detected for the first time by CW-ESR under irradiation without a special cavity or special techniques such as a now method. The structures and properties of the propagating radicals of these conjugated diene compounds were investigated on the basis of hyperfine splitting constants (hfc’s) of the ESR spectra. The spectra were assigned to delocalized allyl-type radicals. Moreover, the mode of addition of propagating radicals to monomer was made clear in the radical polymerization of diene compounds. The propagation rate constants (k(p)) for the radical polymerization of the diene compounds were also determined by using ESR spectroscopy. Values of k(p) for 1,3-butadiene, isoprene, 2-methyl-1,3-pentadiene, 1,3-hexadiene, and 2,4-hexadiene were (150 +/- 40), (125 +/- 30), (35 +/- 10), (20 +/- 10), and (16 +/- 12) M(-1) s(-1), respectively. The difference in these values was reasonably explained by the steric repulsion between the propagating radical and the monomer and by resonance stabilization due to hyperconjugation of the methyl group.