To improve the accuracy of fuel concentration measurements in a direct-injection gasoline engine by LIF (laser-induced fluorescence) technique, two approaches have been couducted. The combination of acetone as the fluorescence tracer of fuel and 266 nm as the excitation wavelength was used for the first approach in order to minimize the error caused by the temperature dependence of the LIF intensity. The second approach was the correction of the equivalence ratio obtained from the raw LIF image for the severe temperature distribution caused by evaporation and superheating of the injected fuel. The temperature distribution in the mixture was calculated ; then the equivalence ratio was corrected for the effects of air density variation and the remaining LIF temperature dependence. This improved techanique was applied to the quantitative analysis of the mixture formation process in a visualized direct-injection gasoline engine both under early and late injection conditions.