Soot emission form the fossil fuel combustion causes many environmental problems and its reduction is one of the most important topics in combustion research. Until now, soot formation of gaseous fuels have been investigated and successfully modeled in many studies. However, soot formation mechanisms of liquid fuels are not fully understood yet. The soot formation mechanism of the liquid fuel is more complex than that of the gaseous fuel because there are vaorization process and interaction between the fuel droplet and flame in the liquid fuel flame. Generally, the soot yield for each fuel at and above threshold is uniquely determined by th flame temperature was achieved. The residence time of the soot particle in the high temperature and there is an inception limit that the residence time is too short to initiate the formation of soot particles.