We investigated the diffusion behavior of polystyrene/poly(2,6-dimethyl-1,4-phenylene oxide) (PS/PPO) nanoparticles mixture prepared by the nanoprecipitation method. The diffusion experiments of liquid PS into the glassy PPO matrix (l-PS/g-PPO) were conducted by annealing the PS/PPO mixture at temperatures between the glass transition temperatures (T(g)s) of the PS and PPO components. By tracing the T-g evolution of the PS-rich domain behind the diffusion front, we obtained the master curve of PS volume fraction during diffusion by time-temperature superposition (TTS) and studied the diffusion mechanism of the l-PS/g-PPO system based on the core-shell model. As there is ongoing debate on the diffusion mechanism for the liquid/glassy polymers interdiffusion, herein we confirm that the diffusion behavior of PS/PPO nanoparticles mixture follows the characteristics of the Fickean mechanism rather than the case II mechanism. Both of the shift factors (a(T)) and the diffusion coefficients in the initial (D-initial) obey the Arrhenius equation, which yield almost the same apparent activation energy (E-df) (about 153.6 kJ/mol). As the PS/PPO nanoparticles mixture is a limited liquid supply system, both of the calorimetric and rheological measurements reveal the departure in the time scaling laws, which corresponds to the change of PS chain dynamics from the reptation type to the Rouse type during the diffusion process.