Entomopathogenic nematodes (EPNs) from the Steinernematidae and Heterorhabditidae families are obligate insect pathogens used as biological control agents against a broad spectrum of key pests. Entomopathogenic nematodes can be considered an attractive biocontrol solution, as they are relatively easy to mass produce, easy to apply and the risk of non-target effects is considered low. The free living infective juvenile (IJ) stage of these nematodes, which is developmentally arrested and well-adapted for long-term survival, carries symbiotic bacteria Xenorhabdus and Photorhabdus in the case of Steinernema and Heterorhabditis, respectively. In a series of laboratory experiments virulence of the three species of Entomopathogenic nematodes (EPNs) including Heterorhabditis bacteriophora isolate IRA10, Steinernema carpocapsae isolate IRA18 and S. feltiae isolate IRA22 were evaluated against developmental stages of the diamondback moth (DBM), Plutella xylostella, to assess their efficacy as biocontrol agents. Steinernema carpocapsae and S. feltiae were tested at six concentrations including 3, 5, 20, 35, 40 and 50 IJs/L and H. bacteriophora efficacy was evaluated at eight concentrations including 10, 15, 25, 35, 40, 55, 60 and 70 IJs/L. The results revealed that developmental stage of DBM larvae profoundly affects virulence of both Steinernema and Heterorhabditis. With significance difference, in both Steinernema species larval mortality reached to its highest rate at 18 IJs/L then fixed at higher IJ concentrations without significance increase. Similar results were recorded for H. bacteriophora, increasing trend of DBM larval mortality was only obvious up to 20 IJs/L. Mortality of exposed DBM pre-pupa to Steinernema species increased up to 35 IJs/PP concentration, in S. feltiae decreasing trend was recorded at higher IJs concentrations. The negative correlation was recorded between LC50 and LT50 values, S. carpocapsae appeared as the most virulent EPN (6.53 IJs/L) on larvae and H. bacteriophora was an effective EPN (6.54 IJs/PP) against pre-pupa. EPNs virulence at dose levels were evaluated by plotting LC50 against ln exposure time and nearly in all data sets deviations from linear model were observed and data were statistically fitted by a two stage phase.