Abstract: Microwave freeze drying experiments are carried out with unsaturated raw beef. The analysis of heat and mass transfer on experimental results shows that the drying time is approximately proportional to initial saturation of porous media. As compared with the calculated results of sublimation interface model, in which sublimation is assumed to take place at an interface, the sublimation - condensation model agrees better with experimental results than those of sublimation interface model. This proves that a sublimation-condensation region does exist during microwave freeze drying of unsaturated porous media.