Storage stability is the one of the most critical issues in polymer-modified asphalt. This study describes the effect of sulfur on the engineering properties of asphalt modified by styrene-butadiene-styrene (SBS). At low sulfur concentration, a continuous phase is present and the composite behaves like asphalt with moderately enhanced elastic response. Appearance of a dispersed structure usually begins at 3% to 5% sulfur. At high sulfur concentrations the compatibility of SBS-modified asphalt is enhanced. Micrographs obtained from a transmission electron microscope illustrate that the addition of sulfur significantly improves the storage stability of SBS-modified asphalt at high temperatures. In-situ chemical reactions resulting from sulfur could reduce SBS's coarseness and enhance solubility of the polymer in the asphalt. The rheological properties of SBS-modified asphalt depend strongly on the sulfur level. Increasing sulfur levels leads to an increase in the complex modulus ratio of SBS-modified asphalt to pure asphalt, and consequently the addition of sulfur improves on the mechanical behavior of SBS-modified asphalt. The sulfur reinforcement becomes pronounced at higher temperature. The engineering properties of polymer-modified asphalt are influenced by the behavior of individual component that forms the material.