Fatty Acid contents of entomopathogenic nematodes(EPNs) were examined by various types of nematodes and culture methods. Seven different types EPNs cultured by in vivo did not contain same fatty acid contents, but similar compositions. It was also found that Steinernema carpocapsae among EPNs cultured by in vivo and in vitro contained not only different fatty acid contents, but also revealed distinctive motilities in a soil. The addition of olive oil in the in vitro culture medium resulted in similar fatty acid contents of S. carpocapsae to in vivo and greatly improved the pathogenicity of nematodes compared to that of soy oil in the medium.