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Surface nitridation of Alumina based composites reinforced with silicon carbide particles and/or whiskers has been studied. The composites processed by hot-pressing were post-treated by HIP process at 1650–1750°C under 150 MPa of nitrogen gas pressure. X-ray diffraction (XRD) analysis indicates that Alumina and silicon carbide on the surface of the composite are converted to aluminum nitride and silicon nitride, respectively, during the post-treatment. Examinations of surface nitrided layer by scanning electron microscopy (SEM) suggest that grain size can be significantly affected by post-treatment condition. Flexure tests indicate that strength increases significantly by the post-treatment. It is discussed that the improvement of mechanical properties included two parts: one came from the densification of sample, the other came from the surface nitrided layer. Specially, the residual compressive stress plays a key role on the improvement of the flexural strength.