Abstract: The effects of heat-treatment on the surface and pore structure of silica gels heated in the range of 200-1100°C have been studied. We have measured the specific surface area by means of B. E. T. nitrogen adsorption method, and the adsorption-desorption isotherms of ethyl alcohol at 25°C by gravimetric method. Moreover, we have calculated the pore volume and the pore-size distributions of silica gels. After heat-treated at 950°C, the specific surface area and pore volume of the purified silica gels are decreased only 25% and 21% respectively, while those of unpurified gels are decreased more than 95%. From these results, it is concluded that the effect of impu-rity is to decrease the thermal stability of silica gel. After heat-treated at various temperatures (200-1100°C), the purified silica gels are studied by infrared spectroscopy in the 2350-4000cm^-1 region (paraffin oil is used as mulling agent). The results show that the absorption bands at 3720cm^-1 (isolated single hydroxyls) are decreased slightly when the temperature is raised from 200 to 950°C, and disappear completely when the temperature is raised to 1100°C. However, the absorption bands in 3300-3600cm^-1 (associated hydroxyls) are decreased much more rapidly than band at 3720cm^-1. The x-ray diffraction shows that only the purified silica gel heated at 1100°C shows the diffraction lines of β-tridymite. When the heating temperature is lower than 950°C,the sintering and thus the decreasing of free hydroxyls are only gentle. However, as the heating temperature is reached to 1100°C,the surface area and pore volume of silica gel decrease markedly, This shows the sintering has occured vigorously, and thus the free hydroxyls may be eliminated almost completely. It is interested to note that after heated at 1100°C the purified silica gel still has quite large surface area of 95m²/g and pore volume of 0.15cm³/g.