Abstract: α-Si₃N₄, β-Si₃N₄ phase contents of silicon nitride ceramics directly effect the properties of silicon nitride materials. Phase fraction of α/β phase transformation of hot pressing Si₃N₄+5wt%Y₂O₃+2wt% Al₂O₃ have been measured by XRD quantitative phase analysis. The effect of hot pressingtemperature and time on silicon nitride α/β phase transformation was studied. The results show the stronge dependance of the α/β phase transformation on temperature. The silicon nitride powder containing 11.0% β phase have been adopted. Under the heating rate of 45℃/min, the pressure of 44.1Mpa, 1450℃×10mins, β phase is 14.7%; 1650℃×10mins, β phase 41.9%; 1750℃×10mins, β phase 95.2%. Starting at 1650℃,α/β phase transfered remarkably. The effect of time on α/β phase transformation varies with temperature. In the range of 1450~1550℃, β phase gradually increases with time. β phase remarkable increase was found at 1550~1650℃. But at 1650~1750℃, the effect of time gradually decreases on α/β phase transformation, when temperature increases Si₃N₄+5wt%Y₂O₃+2wt%Al₂O₃ liquide phase formation temperature (1410℃) have been measured with thermal dilatometer. The SEM observated that the equiaxed a grains transfer to prismatic β grains during the α/β phase transformation at 1650℃.