We have used 2,2'-bis(3,4-carboxylphenyl) hexafluoropropane dianhydride (6FDA)-4,4'-(hexafluoroisopropylidene) dianiline (6FpDA) polyimide synthesized from 6FDA moiety and 6FpDA moiety as a model polymer to analyze and compare different diffusion coefficients of gases to provide a deeper insight into the characteristics of gas penetration through dense membranes. The gas diffusions of N-2, O-2, CH4 and CO2 in 6FDA-6FpDA polyimide dense films were characterized by means of the Henry and Langmuir mode diffusion coefficients D-D and D-H, the average diffusion coefficient D-avg, the local diffusion coefficient (or the effective diffusion coefficient) D-eff as well as the apparent diffusion coefficient D-app, based on the permeation and sorption measurements. The Henry mode diffusion coefficients D-D,D-t obtained from the time-lag method are in fair agreement with that D-D calculated from the permeation and sorption isotherms. Except CO2, the magnitude of D-D, D-H, D-avg and D-app of the three non-interacting gases increases in the order of CH4 D-eff > D-avg > D-app > D-H. However, the deviations among D-eff, D-avg and D-app apparently diminish if the upstream pressure is extremely low or at an infinite diluted situation. While the values of D-eff and D-avg are close to the values of D-D or D-D,D-t under a sufficiently high upstream pressure, as the Langmuir sites are saturated and the Henry mode primarily takes charge of gas transportation.