Copolymerizations of 1,3-pentadiene (PD) with endo-dicyclopentadiene (DCPD) initiated by aluminum trichloride were carried out in toluene. The addition of DCPD in the PD polymerization system does not affect the molecular weight but increases greatly the softening point of the polymer due to the introduction of cyclic structures. The Gardner color scale of the polymer is also raised by the introduction of unsaturated rings of DCPD. The copolymerization gives complete conversions but generates insoluble crosslinked gels at high DCPD contents due to a higher crosslinking reactivity of DCPD than PD. The low-conversion experiments were carried out with small amounts of DCPD in order to determine the reactivity ratio in this copolymenization system (M-1 = PD and M-2 = DCPD). The result of r(1) = k(11)/k(12) = 4.5 demonstrates that DCPD has a lower reactivity than PD toward PD-growing carbocations, and hence, the copolymer shows a smaller DCPD proportion than the corresponding comonomer.