In this study, Sasobit was used as an additive into asphalt. Different ratios of Sasobit (2%, 3%, 4% and 5% by mass of asphalt) were blended into base asphalt Pen. 60/80. The conventional tests, such as penetration test, softening point test, viscosity test, and constituent test, were carried out on asphalt mixed with different additive contents. Attempts to approach the influence of Sasobit on the asphalt micro?structure, the molecular weight distribution and the functional groups of asphalt blended with different additive content were also tested by using GPC and IR methods. The analyses of test results show that properties and micro?structure of Sasobit modified asphalt change dramatically when compared to those of the base asphalt. These changes are related not only with additive content but also with temperature. It is also found that some chemical reactions happen when Sasobit is added into asphalt. With these chemical reactions, the colloid structures, molecular weight distributions, functional groups, and wax crystals of Sasobit modified asphalt will be changed significantly.