In this paper, the influence of heating medium on the thermal diffusivities of fish meat paste products was studied. Model products were heated in boiling water, saturated steam and soybean oil. The differences in temperature raise of the products were interpreted with Biot number of the products. Because of the large overall heat transfer coefficient of heating medium, the temperature raise of the products in boiling water and saturated steam was fast and the Biot number of the products could be recognized as infinite. But the temperature raise of the products in soybean oil was slow and the Biot number of the products was less than 50.