In this study, a novel sintering method, SPS is applied to prepare alpha-Si3N4 ceramics of different relative densities with MgO and AIPO(4) as the sintering additives. The sintering behavior of SPS process is discussed and the factors influencing the relative density of the prepared samples are analyzed. The sintering mechanism is explained by both the reaction between MgO and SiO2 presenting on the Si3N4 powders and solid state sintering of AIPO(4). Microstructures of the sintered samples are observed by SEM images and the phase compositions are analyzed by XRD. The mechanical properties of the sintered ceramics are also investigated, as a function of the relative densities. The resultant alpha-Si3N4 ceramics sintered from 1300 degrees C to 1500 degrees C show fine microstructure with nearly no grain growth and phase transformation and have a relative high bending strength. The alpha-Si3N4 ceramics with relative density of 58-98% and bending strength of 50-420 MPa are obtained. (c) 2007 Elsevier B.V. All rights reserved.