It is crucial to understand how much and how far the structure and dynamics properties of the polymer melt can be affected by incorporating nanoparticles (NPs) in the system. Here, we show that in an athermal all-polymer nanocomposites prepared from mixtures of linear polystyrene chains and cross-linked polystyrene NPs, the influence rang of the NP on the melt polymer properties, namely interphase thickness, depends on the properties investigated. The local segment conformations can be affected in the range of an NP radius. However, if the interphase thickness is defined by the influence range of the NP on the radius of gyration (R-g) of the melt chain, it can be 1.5R(g) for short-chain melts where the chain has a R-g smaller than particle radius R-a. For intermediate chain length polymer melts (R-g > R-g), the influence range of the NP does not change with the chain length. With the chain length further increasing, melt chains cannot feel the existence of the NP; the influence of the NP on the R-g tensors of the melt chain can be negligible. For the dynamics properties, the effects caused by the NP can be differently at different length and time scales. The mobility of the monomers can be slowed down in the vicinity of the NP, but they can have a faster mobility in the tangential direction than in the radial direction of the NP surface. More importantly, the segmental relaxation of the melt chains can be accelerated by the fast thermal deformations of the loose NP surface structures. We demonstrate that the softness of the NP and its deformability are crucial to the dynamics properties of the nanocomposites.