Abstract: The thermal diffusivity is one of the basic paramaters in studying and calculating the heating,freezing,drying and moisture absorbing processes of material. A fast online determination system has been developped which is suitable for agricultural materials,especially those with high moisture content． This paper analyzed the relationship between thermal diffusion coefficient and moisture content of typical materials(i.e. apple and capsicum),and established the mathmatic models showing a nonliner relationship between the thermal diffusion coefficient and moisture content for both apple and capsicum respectively.