The soot emission problem of the diffusion flame around a fuel droplet was taken up and the droplet diameter at the sooting limit,DL, was determined using a high speed camera and an optical method of soot detection. Since no soot emission occurs from the flume if the droplet diameter is smaller thanDL. the magnitude ofDLmight be an index to the sooting tendency of liquid fuels, just like the fuel flow rate at the sooting limit which is a useful index to estimate the sooting tendency of gaseous fuels. The effects onDLof fuel type, fuel blending, and surrounding air velocity, as well as time variations of droplet diameter and flame height during combustion, were investigated. The main findings were: (1)D, was increased and the sooting tendency was lowered with the increase in the air velocity although it promoted fuel evaporation and soot formation; (2) When two types of fuels with different sooting tendencies were blended, the concentration dependence ofDLwas greatly affected by the relative volatility between the component fuels.