Abstract: The sublimation and condensation phenomena caused by vapor movement in the pores partially filled with ice during microwave freeze drying of unsaturated porous media is studied. By the use of a sublimation-condensation model developed by the authors in a previous paper,the numerical simulation of heat and mass transfer process for frozen beef under microwave heating is made. The results show that the saturation of ice in sublimation-condensation region would decrease greatly during the microwave freeze drying.The calculation also indicates that the influence of sublimation-condensation region is significant and should not be neglected in microwave freeze drying process as compared with the numerical results of a sublimation interface model under the same operation condition.