Gravimetric technique was used to understand and classify the effect of fluorine group on water diffusion characteristics and morphology of polyimide thin films, two different families of copolyimides; flexible, BTDA-ODA and rigid, PMDA-PDA. The sorption kinetics of water in these films was well fitted to Fickian diffusion. In BTDA/6FDA-ODA polyimide thin films, the diffusion coefficient and total water uptakes were varied with increase of the mole fraction in fluorine group from 14.8x10^-l0cm²/sec to 16.9x10^-l0cm²/sec, 2.78 wt. % to 1.70 wt. %, respectively. And in PMDA/6FDA-ODA polyimide thin film, the diffusion coefficients and total water uptakes varied with increase of mole fraction in fluorine group from 7.3x10^-10cm²/sec to 12.8x10^-l0cm²/sec, 4.33 wt. % to 3.18 wt. %, respectively. The effect of fluorine group on sorption kinetics is larger in rigid structure, PMDA-PDA than that does in flexible structure. BTDA-ODA polyimide morphology is not very affected by incorporation of fluorine group. But, rigid PMDA-PDA polyimide morphology was affected by the fluorine group, which increases the flexibility of polyimide structure. The diffusion coefficient and sorption kinetics were affected by the chemical structure of polyimide thin film.