The effects of dosage(by mass) of polyphosphoric acid(PPA) on physical and rheological properties of asphalts(TZ 70, SK 90) were investigated, the changes of chemical composition and colloid structure of the asphalts were studied by chemical composition and atomic force microscopy(AFM) analyses. The results show that the softening point, viscosity, complex modulus and rut factor of the asphalts are increased with the increment of dosage of PPA, while the ductility, penetration and phase angle of the asphalts are decreased with the increment of dosage of PPA. Compared with TZ 70, the physical and rheological properties of SK 90 are improved more obviously. The thermo stability of the asphalts is improved with the introduction of PPA. The thermo stability of TZ 70 is more obviously improved than that of SK 90. After PPA added, the transformation of asphalt from resins to asphaltenes is promoted, which contributes to the association of the dispersed phase and increases the stiffness of asphalt. As a result, the physical and rheological properties of the asphalts are improved remarkably.