The classical Fick's law with a constant coefficient is not suitable for gas diffusion in fractal porous media. This study analyzes non-steady-state gas diffusion through a fractal porous media with fractal pore models used to create natural porous media. The governing equations for the molecular motion are then used to model the molecular diffusion in the fractal pore model. Extensive numerical simulations are then used to develop a general diffusion equation. The predictions of the diffusion equation agree well with numerical simulations and experimental data. The diffusion equation reproduces the classical Fick's law when the fractal pore structures have a weak effect on the diffusion. The results show that non-steady-state diffusion depends on the history of the diffusion process and that the diffusion rate increases with increasing porosity, decreasing specific surface area, and decreasing fractal dimension.