Triglycerides and fatty acids compositions in the juveniles of two species of Heterorhabditis (Heterorhabditis bacteriophora and H. indica) and one species of Steinernema (Steinernema riobrave ) were analysed and determined at two ages 0 and 20 days for S. riobrave and H. indica , less than 7 and 30 days for  H. bacteriophora. Results revealed that triglycerides comprised approximately 82, 88and 85 % of the total lipids in S. riobrave, H. indica and H. bacteriophora respectively, Fatty acids recorded 8, 7 and 8% of the total lipid content, so, neutral lipids are the major content of total lipids. The major fatty acids found in nematode juveniles of three species were: palmitic C16:0, stearic C18:0, oleic C18:1 and linoleic C18:2. The most abundant saturated fatty acids were palmitic (C16:0) and stearic (C18:0) acids. Oleic (C18:1) and linoleIic (C18:2) were the principal unsaturated fatty acids. These nematodes have relative high levels of poly unsaturated fatty acids. Triglycerides and fatty acids composition were declined with storage. This decline was quantitative rather than qualitative. These results might be helpful in increasing the efficiency of entomopathogenic nematodes (EPN) as biological control agents against insect pests and improving different nematode species or strains. Fatty acids may be play role in toxicity of entomopathogenic nematodes (EPN) against hosts.