Armyworms are polyphagous insect pests of agricultural crops. Their population is usually managed below threshold level using insecticides. However, a single control measure is insufficient to attain a sustainable pest management. Hence, this research was conducted to assess the three species of entomopathogenic fungi as potential biological control agents against Mythimna separata (Walker) or paddy armyworm. Dose mortality assays were conducted to determine the pathogenicity of Beauveria bassiana (Bals.) Vuill., Metarhizium anisopliae (Metschn.) Sorokin, and Metarhizium rileyi (Farlow) Kepler, S.A. Rehner and Humber against various biological stages of M. separata including egg, 1st to 6th larval instars, prepupa, and pupa. These entomopathogenic fungi did not have ovicidal activity but affected the survivorship of the resulting neonates. Exposure of eggs to B. bassiana, M. anisopliae, and M. rileyi infected 64, 60, and 70% of the resulting neonates, respectively. Adult emergence was also reduced in fungal-treated (29% in B. bassiana, 10% in M. anisopliae, and 10% in M. rileyi) as compared with 64% in control. At 7 d after treatment, M. rileyi applied at 1 x 105 to 1 x 109 conidia/mL was the most pathogenic causing a mean larval mortality of up to 70% as compared to B. bassiana and M. anisopliae with less than 10% mortality. Among the larval instars, the 1st, 2nd, and 3rd were the most susceptible to fungal infection. Conidial concentrations (1 x 108 and 1 x 109 conidia/mL) induced the highest larval mortalities as compared to the lower conidial concentrations (1 x 105 and 1 x 107 conidia/mL) of the entomopathogenic fungi. The prepupal and pupal stages were also slightly susceptible to the entomopathogenic fungi. These findings showed the pathogenic effect of the entomopathogenic fungi particularly M. rileyi to M. separata.