In order to design various microwave heating apparatuses, it is necessary to measure the transforming rate such as cooking, drying rate and so on, and establish the transforming rate equation. In previous papers (KUBOTA et al., 1988; KUBOTA et al., 1990; Ru et al., 1990), we have studied the transforming rate equations of microwave heated cooking and drying of potato and so on. The browning phenomena, flavor change and so on according the temperature rises of food decrease the commercially value. The vacuum operation is useful to decrease the temperature rises. In this paper, we studied the rate equation and the drying phenomena of microwave vacuum drying of potato.