In gasoline direct-injection (GDI) engines, atomized spray is desired to achieve efficient mixture formation needed for good engine performance because the injection process leaves little time for the evaporation of fuels. Therefore, a substantial understanding of global spray structure and quantitative characteristics of sprays are decisive technology to optimize combustion systems of GDI engines. In this study, the spray characteristics and atomization mechanism of the high-pressure swirl-type GDI injector were analyzed using a phase Doppler anemometer (PDA) system, and the different breakup mechanisms controlling the droplets with various sizes were investigated. In addition, the measured droplet size data were compared with Nukiyama-Tanasawa and Rosin-Rammler distribution functions. From experimental results, the detailed spray structure and factors influencing the atomization mechanism of fuel droplets were investigated with the classification of droplet size. The results also revealed that the Nukiyama-Tanasawa distribution function was found to overestimate the population of small drops and Rosin-Rammler function overestimated the population of large drops.