The addition copolymerization of 5-norbornene-2-carboxylic acid methyl ester (NBCM) with norbornene (NB) catalyzed by CpNi-based (Cp = eta(5)-C5H5) systems, i.e., CpNi(PPh3)Me/B(C6F5)3 and CpNi(PPh3)Cl/AlMe3/B(C6F5)(3), was studied with particular focus on the reactivities of the endo-versus exo-isomers of NBCM. Unlike the well-known higher reactivity of the exo-isomer in Pd-II-catalyzed polymerizations, the CpNi system showed a small but noticeable preference for the endo- isomer. The better reactivity of the endo-isomer is more pronounced in the copolymerization of cis-5-norbornene-2,3-dicarboxylic acid dimethyl ester (NBCD) with NB, where only the endo-isomer gave the copolymer when the NB/NBCD feed ratio was 6/4.