This paper presents a laboratory study in styrene-butadiene-styrene (SBS) polymer modified asphalt including the dispersion process of SBS in asphalt and the asphalt performance without chemical reaction. Fluorescence microscope was adopted to characterize the dispersion of SBS in asphalt. The effects of various factors on modified asphalt were studied by means of the SBS particle size distribution, which was obtained by micrographs using the Image-Pro Plus image analysis program. The fundamental characteristics of SBS modified asphalt samples were determined using conventional test methods. The test results indicate that higher shear rate and longer shear time could promote the SBS dispersion in the low range, but there exists a critical shear rate and shear time. The higher shear temperature could improve the dispersion of SBS in asphalt. When the shear temperature increases to a certain number, the SBS particle size decreases, which do have adverse effects on the performance of SBS modified asphalt. With the content of SBS increasing, the particle size distribution becomes wider and the softening point of the modified asphalt increases, while the penetration and ductility of modified asphalt decrease. When the content of SBS is constant, as SBS particle size decreases, the softening point and viscosity increase, penetration decreases, and the ductility of SBS modified asphalt first increases slightly then decreases. However, when the content of SBS varies, as the particle size of SBS decreases, the softening point and viscosity of SBS modified asphalt increase, while penetration and ductility of SBS modified asphalt decrease.