The minimal inhibitory concentration of cinnamon bark oil extract against pathogenic Streptococcus sp. was 250 ppm. An analysis of the extract by gas chromatography and mass spectrophotometer showed three active substances, cinnamaldehyde (C9H8O) at 83.1 %, coumarin (C9H6O2) at 12.6 % and cinnamic acid (C9H8O2) at 2.2 %.An effect of cinnamon bark oil extract on growth and resistance to Streptococcus sp. in sex-reversed red tilapia after feeding the extract-supplemented diets for 8 week was investigated. Four experimental diets were formulated to contain the extract at 0, 250, 500 and 1,000 ppm. The fish fed diet supplemented with 250 ppm cinnamon bark oil extract showed the best feed conversion ratio (p 0.05), fish fed 250 ppm extract-supplemented diet showed the highest resistance to Streptococcus sp. After six weeks, the fish fed diet supplemented with cinnamon oil extract at 1,000 ppm showed abnormal hepatic cells such as atrophy and degeneration and hepatic sinusoids.