The effect of radiation intensity and thickness of slab on the moisture diffusion characteristics of potato during FIR drying is investigated. The standard solution to the non-stationary state diffusion equation was used as a mathematical tool. A model fitting procedure was applied to the experimental drying data to determine the diffusion coefficients. The diffusivity was found to vary with radiation intensity. The diffusivity also increased with the thickness of slab at a constant radiation intensity level. It has been indicated that decrease in energy of activation caused an increase in the drying rate. The diffusion coefficient values predicted with the proposed model were in close agreement with the experimental.