In this paper, we report the molecular weight dependence of the crystal growth of isotactic polystyrene from 10 nm ultrathin films. The growth rate and characteristic length of the branching morphology of a crystal grown in 10 nm ultrathin films change depending on the molecular weight of the sample. Analysis of the molecular weight dependence according to the theory of growth front instability reveals that the diffusion coefficient of molecular chains in ultrathin films around branching crystals scales with the molecular weight as M-w(-1.4) for samples with weights higher than the critical molecular weight for the entanglement of polystyrene. This result indicates that the polymer chains in the depletion zone around the crystals diffuse in quasi-two dimensions through the reptational motion modified on an attractive substrate.