Unlike low-molecular-weight inorganic or organic nucleating agents (NAs), the physical properties, including phase transition and rheological behaviors, of polymeric NAs are greatly influenced by molecular weight. In this paper, the molecular-weight dependence of the beta-nucleation behaviors of a polymeric NA (a main-chain liquid crystalline (LC) polyester, PBDPS) was studied in detail. The nucleation effect of PBDPS on isotactic polypropylene (iPP) shows a strong molecular-weight dependence. Low-molecular-weight PBDPS tended to induce the formation of alpha-iPP, whereas high-molecular-weight PBDPS tended to induce beta-iPP. Moreover, the relative content of beta-iPP crystals (k(beta)) first increased and then decreased as the molecular weight of PBDPS increased, with the maximum value (86.1%) obtained when 2 wt % PBDPS21.2k was incorporated. Furthermore, the stiffness, toughness, and heat distortion temperature (HDT) of iPP were simultaneously improved by incorporating PBDPS.