The crystallization behaviors of three types of polymer systems grafted on a flat solid substrate with different grafting densities were investigated by dynamic Monte Carlo simulations. For both of the systems with low and medium grafting densities, nucleation induction period becomes shorter with the increasing polymer-substrate interactions. However, the nucleation mechanisms are different. For the systems with low grafting density, the increase of the attractive interactions results in the enhancement of heterogeneous nucleation process of grafted polymers. For the systems with medium grafting density, the attractive interactions can compensate for the conformational entropy loss induced by the restriction of the substrate, resulting in the increase of local segment density near substrate surface and the improvement of nucleation ability. Meanwhile, nucleation mode changes from intermolecular fringed-micelle nucleation to intramolecular chain-folding nucleation with the increasing interactions. For the systems with high grafting density, crystallization behaviors are almost not affected by polymer-substrate interactions. These findings are helpful to reveal the microscopic mechanism of crystallization behaviors of polymer nanocomposites and the corresponding reinforcement mechanism. (C) 2017 Elsevier Ltd. All rights reserved.