Abstract: A mathematical model of freeze-drying process was developed for the biological materials. The model described here took into account the fractal feature of the dried layer. The continuity equations for the water vapor and the inert gas in the dried layer used the diffusivity equation of the fractal porous medium, and the Knudsen diffusivity decreased exponentially with the dried layer thickness. The model was capable of predicting the influence of the fractal feature of the dried layer on the motion of the sublimation interface and the drying time of the freeze-drying process of biological materials. The model could also predict the temperature profile and the concentration of bound water profile in biological materials during the freeze-drying process. The freeze-drying process of spirulina in glass vial was simulated using fractal model by software Matlab and Fluent. The simulation results agree with the experimental data.