Abstract: To explore the pathogeny of suspected Tilapia Streptococcus broken out in a fish farm in Foshan, Guangdong province, bacteria in the liver and gallbladder tissue of diseased Tilapia was isolated with rainheart infusion medium and Tryptic soy agar medium. The Gram positive Streptococcus was acquired and identified by 16S rDNA primer gene sequence, then the gene evolution was analyzed and drug sensitive test was examined with 10 kinds of antibiotic including azithromycin, streptomycin, amikacin, etc. The results showed that the isolated strain was identified as S. iniae by 16S rDNA gene amplification. Gene evolution results showed that the identified Tilapia S. iniae was the most close gene relationship with the reported Streptococcus and poultry vice milk Streptococcus. The drug sensitivity test result showed that the strain was moderately sensitive to azithromycin and cefotaxime, and not sensitive to other antibiotics.