With ultrasonic wave transmission-reflection combined method, measurement of compressional wave velocities (v_p) of bojite was carried out up to 1 085℃ at the pressure of 0.6 GPa, 1.0 GPa and 2.0 GPa respectively. Based on investigating the partially molten products, the authors calculated the v_p of bojite at different pressures and temperatures with volume percentages and elastic parameters of minerals and melt. The calculations show the same v_p-t correlations with experiment runs, which indicate that at first v_p decreases slowly with increasing temperature, and then decreases quickly above about 800℃ to 900℃ at different pressures. The investigations of the products indicate that bojite is dehydrated and partially molten when the temperature is higher than 812℃ at 0.6 GPa, 865℃ at 1.0 GPa and 919℃ at 2.0 GPa respectively. It is believed that partial melting is responsible for quick decrease of v_p at high temperature. There is more quick decrease of v_p of bojite when volume percentages of melt are lower than about 2%. It is thus held that water and melt with low viscosity result in the sharp decrease of v_p, because it is easy for them to move into the microfissures in nature rock at the beginning of dehydration and partial melting.