Many efforts have been devoted to understand structure-property relationships of polymer-modified asphalts in order to improve their performance, but the acquired knowledge is still incomplete and never enough. The objective of this study was to optimize molecule structure parameter (vinyl acetate content) of ethylene-vinyl-acetate copolymer (EVA) for asphalt modification. The results indicated that viscoelastic functions (G′, G″ and G∗), viscosity and the percent recovery at high temperatures of modified asphalt present the maximum value, corresponding to sample with 18wt% VA, as the increase of VA in EVA. While these functions decrease significantly when VA is more than 18wt%. With respect to low temperature performance, the faster relaxation, lower stiffness and greater m-value are observed for samples containing higher VA contents, indicating enhanced cracking resistance. Furthermore, better stability corresponds to sample with higher VA. In microscopy, polymer-rich phase tend to be smaller in size with the increase of VA content. Besides, the distribution of EVA particle with higher VA in asphalt tend to be sparse and also be inconspicuous. Consequently, the moderate VA content, 18wt% in this study, is pivotal to acquire the ideal high as well as low temperature performance and to balance the viscoelastic properties and stability.