Abstract: The low-temperature performances of 90# asphalt，nano-silica modified asphalt and hydrophobic nano-silica modified asphalt were comparatively evaluated with low-temperature performance test of conventional asphalt and bending test of small mortar beam in order to investigate the effect of the hydrophobic nano-silica on the low-temperature performance of asphalt binder． Scanning electron microscope test were conducted to analyze the chara-cteristics and modification mechanism of the two kind of modified materials． Then the Fourier transform infrared spectroscopy test was conducted to study the function mechanism of modified asphalt to improve low-temperature performance of asphalt． Ｒesearch results show that: the penetration index of hydrophobic nano-silica modified as-phalt is the highest; its equivalent brittle point is 80% lower than that of nano-silica modified asphalt; its maxi-mum flexure tensile strain is 80% higher than that of nano-silica modified asphalt and 350% higher than that of ma-trix asphalt at －15℃; its ultimate flexural tensile strength is 69% higher than that of matrix asphalt at －15℃，which indicates that the low temperature performance of hydrophobic nano-silica modified asphalt is excellent． Hy-drophobic nano-silica particle is 11% smaller than nano-silica particle，and the size distribution of hydrophobic nano-silica particle is more uniform． The surface of hydrophobic nano-silica particle is organic． Compared with nano-silica particle，the surface of hydrophobic nano-silica particle is rougher． Hydrophobic nano-silica particle is not easy to gather when modified mixing． The three-dimensional mesh structure formed by modified particles and asphalt improves the low-temperature characteristics of asphalt． The C—O chemical bond is formed between hydro-phobic nano-silica and asphalt，which enhances the bond between modified particles and asphalt，and the strong chemical bond shortens the length of asphalt tensile chain which results in an increase in the deformation ability．

Key words: pavement engineering, hydrophobic nano-silica modified asphalt, microanalysis, low-temperature performance, influence mechanism