Several types of oil dispersant chemicals have been used worldwide to remove and reduce the effect of oil spill to marine environments and most of the countries are following the rules and regulations for the application of chemical dispersants. Since, there are no records of use of chemical dispersant in Sri Lanka it is necessary to study the toxicity of these chemical dispersants to marine life. Therefore, the manuscript explain an experimental design to determine the acute toxicity of Adt concentrate type 3 oil dispersant chemical to marine fish, Abudefduf vaigiensis under laboratory condition and the LC50 values for the used chemical according to probit software. The results of the above research will be useful for future researches in the field of toxicology and the people who are involving in the marine pollution prevention sector mainly in developing countries.