Abstract: Tilapia is an important species in freshwater aquaculture and widely cultures in tropical and subtropical regions of many countries. It is famous for rapid growth, well-adapted, fast-breeding and higher disease resistance. Like other large fish, they take a good supply of protein for human and are very popular among artisanal and commercial fisheries. However, with the unscientific husbandry, environmental degradation and other reasons, tilapia is no longer a disease-resistant species. There are a number of bacterial and viral diseases, and many of them are devastating, such as Streptococcus and Edwarsiella tarda. So understanding and identifying the new causative agent of different disease is significant for the future culture of tilapia. In this study, we firstly isolated two Aerococcus spp. Mnlv1 and Mnlv2 from diseased tilapia in Maonan of Guangdong Province. With a series of experiment, such as bacterial morphology, condition comparison of culture, physiological and biochemical property and 16S rRNA gene analysis, the two strains were identified as Aerococcus viridans. Moreover, in order to study the pathogenicity and aetiological condition of this organism, we employed three different artificial pathways to infected tilapia. In addition, twenty-nine antibiotics were selected to test the sensitivity to Aerococcus viridans. The results showed that Aerococcus viridans was pathogenic to tilapia but it generally could not infect healthy tilapia actively. It likely to be a conditional pathogen and the environmental stress could enhance their infection to tilapia. The drug sensitive detection showed that these two Aerococcus strains were hypersensitive to quinolone group (norfloxacin, Ofloxacin, enrofloxacin, nalidixic acid, Lomefloxacin, enoxacin, Fleroxacin), part lactam group (ceftriaxone, cefaclor, Cefazolin, cefobis) and part Aminoglycosides group (Amikacin, Tobramycin, gentamicin) and minocycline, mid-sensitive to midecamycin, Spectinomycin and neomycin, resistant to part macrolides group (erythrocin, acetyl spiramycin, roxithromycin), part lactam group (cephalothin, cefalexin, amoxicillin, oxacillin, penicillin, ampicillin) and tetracycline.