Ring-opening metathesis polymerizations of N-4-substituted benzyltetrahydrophthalimide (RBnTPI-R') (4-substituent on N-benzyl group; R = H, COOMe; (M), COOH; (C), NO₂; (N), substituent on ring-fused carbons; R' = H or Me) in the presence of the 1st Grubbs' ruthenium catalyst (1.0~4.0 mol%) were carried out to investigate the relationship between polymerizability and structure of cyclohexene ring. The yields and number-average molecular weights (M_n) of poly(RBnTPI-H) having 4-subsutituent on N-benzyl group were lower than unsubstituted poly(HBnTPI-H). But the yield and M_n of poly(MBnTPI-H) obtained under bulk conditions were higher than those obtained in solvents. Poly(NBnTPI-H)s obtained with the 1st Grubbs catalyst (4.0 mol%) had much higher yield (71 %) and M_n (2,200) than polymers obtained with the Grubbs catalysts (1.0 mol%). The polymerizability of RBnTPI-Me having bulky methyl groups on ring-fused carbons was expected to be improved by the enhanced strain of cyclohexene ring. The M_ns of poly(RBnTPI-Me) were higher than those of poly(RBnTPI-H) remarkably. In particular, the polymerization of CBnTPI-Me in THF at 50 °C produced the polymer with the highest M_n (25,800). The polymerizations of the trans:cis mixture of CBnTPI-H gave only the poly(trans-CBnTPI-H). This suggests that the ring-opening polymerizability was dependent on the strain of cyclohexene due to the neighboring rigid plane structure of a trans-imide ring. The improvement of polymerizability was attributed to an increase of the strain of cyclohexene due to the neighboring rigid plane structure of a trans-imide ring and bulky methyl groups on ring-fused carbons.