By employing molecular dynamics simulations, we explored different size-dependent dynamics of NPs in semiflexible linear and ring polymer nanocomposites. Large NPs diffuse faster slightly in flexible ring polymer nanocomposites than in flexible linear polymer ones. However, the motions of large NPs in semiflexible ring polymer nanocomposites with large bending energies are very slow and NPs exhibit the glassy state in which the NPs are all caged by the neighbouring semiflexible ring polymers, while large NPs in semiflexible linear polymer nanocomposites transport much faster than those in semiflexible ring polymer ones because rod-like structures of semiflexible linear polymers can favour the motions of NPs. Meanwhile, the diffusion coefficients D-polymer of semiflexible ring polymers decrease monotonously with increasing bending energy K-b, while the minimum diffusion coefficient for semiflexible linear polymers occurs at the moderate bending energy such as K-b = 6, in which the semiflexible linear polymers hold the maximum entanglement density. NP size, polymer topological constraint as well as chain stiffness affect the dynamics of NPs in polymer nanocomposites seriously, and this investigation can help us understand the complicated dynamical behaviours for NPs in polymer nanocomposites. (C) 2017 Elsevier Ltd. All rights reserved.