The hot isostatic pressing (HIP) of Si3N4 was performed in the temperature range 1500-1800°C and within the pressure range 90-172 MPa. Both additive-free Si3N4 and Si3N4 with Y2O3 additions were studied. During the HIP process the transformation of α-Si3N4 to β-Si3N4 occurs and the amount of α→β transformation increases as the HIP temperature and HIP pressure increase. The α→β transition also depends on the Y2O3 content. The density of the Si3N4 consolidated by HIP increases as the HIP pressure, HIP temperature and Y2O3 content increase. The hardness and Young's modulus of the product increase gradually with increasing Y2O3. The effect of outgassing the starting material prior to HIP on the properties of the final product is discussed.