Gut symbiotic microorganisms of insects are known that play important roles in the digestion, development, survival, and reproduction of host insects, and it is widely accepted that symbionts synthesize essential amino acids or vitamins for host insect among these relationship. More recently, however, it has become increasingly apparent that symbiont's contribution to their host are not limited to nutrition. Actually, these symbionts are frequently observed near an insect's digestive tract. In this study, we isolated and identified gut bacteria of ladybird, Harmonia axyridis, which is a predator and used the factor for biological control of aphids, and was aimed to survey gut bacteria affecting to ladybird's biological or ecological characteristics. We separated the guts of larvae and adults ladybird and cultured them in 4 media, such as NA, skim-milk, CMC and CMA. About 33 isolates including genus Arthrobacter were identified in this study. These were cultured in NA media with several antibiotics. We selected ofloxacin and penicillin that inhibited these bacteria growth in media, and reared ladybirds as prey with aphids treated ofloxacin and penicillin 1000ug/1ml concentration. After that, weight of pupae, larval stage, emergence stage, fecundity and hatching rate were investigated. It was shown that, ofloxacin and penicillin treatment was caused by weightless of pupae, fecundity and delay of larval stage, and the affected hatching rate as well.