We investigated in situ surface modification of SiAlON ceramics by sintering powder compacts in packing powder with chemical compositions different from those of the compacts. For beta-SiAION, it was possible to make an alpha- or O'-SiAlON layer on its-surface. The mechanism of this in situ surface modification was attributed to a liquid flow from the packing powder to the compact, and reactions between the compact and the infiltrated liquid. It appeared that the new phase formation at the compact surface was determined by the amount and composition of the infiltrated liquid. The surface modified beta-SiAlON ceramics showed improved wear and oxidation resistance, suggesting that the surface modification technique can provide opportunities for improving the mechanical properties of monolithic SiAlON ceramics.