The occurrence of secondary atomization in emulsified fuel is widely accepted to explain a decrease in NO(x) and soot emissions. Although it is known that secondary atomization, such as puffing and micro-explosion, occurs in heating process, little detailed information is available on the secondary atomization behavior of emulsified fuel droplets. In this paper, the effect of surfactant on secondary atomization was investigated. A method was proposed to visualize the internal behavior in emulsified fuel droplets using colored water. Emulsified fuel droplets prepared using various surfactants and colored water were suspended by use of a thermocouple, and the secondary atomization behavior of the droplets was observed using a hi-speed video camera. In the case of O/W emulsified fuel droplets, reaggregation of dispersed water droplets and micro-explosion were observed after aggregation of dispersed oil droplets and phase inversion from O/W type to W/O type. Although the probability of micro-explosion occurring decreased with an increase in HLB value in the case of W/O emulsified fuel droplets, little difference was observed in the probability as a function of HLB value in the case of O/W emulsified fuel droplets.