Mass transfer, diffusion, adsorption and photochemistry are figured out to be the controlling mechanisms for photocatalytic degradation of volatile organic compounds (VOCs) using nano-TiO 2 catalyst. A mathematical model for the non-steady photocatalytic degradation process is developed by incorporating these mechanisms in a plane plate air purification physical model. The governing equation and boundary conditions are solved using finite difference method. The computation results are validated using the data obtained from the experiments. And then the model is employed to study the effects of some key factors on the degradation rate of formaldehyde. These factors include UV light intensity, UV light attenuation coefficient, adsorption, catalyst thickness, diffusion, and flow rate. The model can be employed to assist the design of VOCs photocatalytic degradation system using nano-TiO 2 catalyst.