By using the diamond-anvil pressure cell coupled with resistant-wire heating and the six anvil pressure vessel, the phase transformation of AlPO4 has been investigated at pressures up to -17.8 GPa and at temperatures from 500℃ to 1700℃. Under atmospheric pressure, the transition temperature from berlinite (qurtze-type AlPO4) to tridymite-type structure is about 778℃, and from tridymite-type to cristobalite-type structure-about 1100℃. Theα-β transition of berlinite takes place at 586℃. Berlinite is stable at pressures below 5.5 GPa and temperatures below 1300℃. It decomposes into the mixture of Al2O3 (Corundum) plus P2O5 at 5 GPa and over 1700℃. Tridymite-type AlPO4 is found to transform to a new phase at 6 GPa and 500℃. The new phase has been indexed on the basis of an orthorhombic cell with lattice parameters of a=4.930, b=7.200, c=6.165Å at room temperature and atmospheric pressure. At pressures greater than 11 GPa, AlPO4 transforms from berlinite or tridymite-type to NiSO4-type form. A possible P-T phase diagram of AlPO4 is given in this paper.