The purpose of the study was to determine the effect of mixing rate of PET materials with SiO2 on the particle distribution state, to determine the effect of the treatments on the crystalline structures, thermal and mechanical properties of the mixed materials, and to determine the effect of surface treatments of SiO2 with the silane coupling agents on the surface properties of mixed materials. Measurements were carried out with Image analyzer for particle distribution state, X-ray diffractometer for the crystallinity, DSC for thermal properties and Instron for mechanical properties, respectively. According to results the following conclusions could be made : 1) Relatively uniform panicle distribution of the mixed materials was obtained by melt blending after the fillers were mixed with the PET powder sufficiently. 2) Using the particle size of the fillers which was remarkably greater than that of the crystallite, the crytallization of the polymer was hindered by the excess content of the fillers 3) Tm of the mixed materials was lowered with increasing of SiO2 content, and their tendency was more notable when mixed materials were treated with silane. 4) The initial modulus of mixed materials was increased with SiO2 contents, and the mixed materials became brittle and so tensile strength of them was decreased with increasing SiO2 contents.