Abstract: It is proved that Chromolaena odorata L. is usable natural antibacterial agent to fight Vibrio harveyi. The study aims to isolate and identify bioactive compound that is responsible for being natural antibacterial agent to fight Vibrio harveyi on post-larval tiger prawn (Penaeus monodon). The study has worked to isolate 5 fractions of bioactive compound. Based on the antibacterial activity testing to the four fractions, fraction four is evidently in charge of being antibacterial agent as it has the highest capability than other fractions. Based on the identification of Fraction 4 through FT-IR, UV-Vis and GC-MS methods in comparison to the present literature, the bioactive compound falls within flavonoid compound as it is identified as quercetin.