X-ray diffraction patterns, IR spectra and zeta-potentials of silicon nitride and $eta$-Sialon powders were investigated before and after surface manipulations. $eta$-Sialon powder was produced from Hadong Kaolin by the carbothermic reduction and simulataneous nitridation. Isoelectric points of as-prepared Si3N4 and $eta$-Sialon powders were 8.4 and 7.4, respectively. After both silicon nitride and $eta$-Sialon powders were oxidized at 80$0^{circ}C$ for 24 h in air, the isoelectric points of these powders corresponded to that of silica (pH=3). I case of the addition of Darvan C as deflocculant, its isoelectric point was 3 and zeta-potential was nearly constant in the range of pH 5~12. When SN 7347 was used as deflocculant, its isoelectric point was 8.3 and zeta-potential over -156 mV was measured above pH 11.