Fundamental properties and surface treatment effects of domestic kaolin calcined at higher temperature were studied to develop reinforcing fillers for rubbers. The results obtained are as follows: (i) X-ray diffraction and scanning electron microscopy studies revealed kaolinite as a major constituent of the raw kaolin used in this study. (ii) Physical properties of natural rubber vulcanizates compounded with the calcined kaolin fillers treated with poly(maleic anhydride) and sodium polyphosphate are favorably improved. Particularly, the kaolin filler treated with sodium polyphosphate(designated as PT series) shows excellent physical properties compared with hydrated silica. PT Series Hydrated Silica Tensile strength, $kg/cm^{2}$ (average) 182 132 Elongation, % (average) 629 587 300% Modulus, $kg/cm^{2}$ (average) 65.6 55.0