The solubility and diffusivity of CO2 in polymer matrix play crucial roles in controlling the nucleation and growth of bubbles in the polymer/CO2 foaming process. In this work, the effects of polymeric aggregation states (solid and melt states) and layered filler with one-dimensional nanostructures on the solubility and diffusivity of CO2 in isotactic polypropylene (iPP)/nanomontmorillonite (nano-MMT) composites were investigated. The results show that the solubility in the iPP composites has little change with an increase of nano-MMT concentration while the diffusivity of CO2 decreases with increasing nano-MMT content. Considering the effects of a one-dimensional nanostructure filler and a polymeric crystalline region on the free volume and the diffusion path, two models on the basis of the free volume theory were established and used to well correlate the experimental diffusion coefficients of CO2 in melt-state iPP composites and predict the CO2 diffusivity in solid-state iPP composites.