We synthesized a new series of MJLCPs with a polynorbornene main chain, PNbnPT (n = 8, 10, 12, 14, 16, 18, which is the number of carbons in the side-chain alkyl tails), through ring-opening metathesis polymerization and investigated their phase behaviors. The monomers were facilely synthesized in satisfactory yields through a key intermediate, N-(2,5-dicarboxylphenyl)-cis-5-norbornene-exo-2,3-dicarboximide. Differential scanning calorimetry, polarized light microscopy, and wide-angle X-ray diffraction results demonstrate that the phase behavior of PNbnPT is strongly dependent on the alkyl-tail length. Polymers with relatively short alkyl tails, PNb8PT and PNb10PT, are amorphous in the whole temperature range before degradation. When the alkyl-tail length increases, PNbnPTs (n = 12, 14, 16, 18) develop into the smectic A phase, and the degree of order increases with increasing alkyl-tail length. The synthetic pathway for MJLCPs with a polynorbornene main chain in this work is more robust and greener than those reported to the best of our knowledge. And MJLCPs with a polynorbornene main chain show quite different phase behaviors compared with those with a polyethylene backbone, and they may be used to create fascinating functional materials.