A numerical investigation of air-fuel mixing in gasoline direct-injection (GDI) engines is presented in this paper. The primary goal of this study is to demonstrate the importance of fuel representation. In the past studies, fuel has been usually modeled as a single component substance. However, most fuels are mixtures of hydrocarbons with diverse boiling points, resulting in mixture vaporization behavior substantially different from single-component behavior. This study presents a newly developed multicomponent vaporization model, which takes into account important mechanisms such as preferential vaporization, internal circulation, surface regression, and non-ideal behavior in high-pressure environments. A sheet spray atomization model was also used to calculate the disintegration of the liquid sheet and the breakup of the subsequent droplets. The results of a single-component fuel representation and a multicomponent fuel representation were compared. The multicomponent fuel used consists of four components and has a distillation curve similar to that of a California Phase II gasoline. It was found that fuel representation affects spray penetration, impingement behavior and global vaporization history. The magnitudes of these effects depend on injection timing. It was also found that wall temperature has strong effect on fuel vaporization. Hot wall significantly promotes film vaporization, and cold wall slows down film vaporization, especially in the early injection.