The present investigation was achieved at Sakha Agricultural Research Station during 2008 and 2009 seasons to evaluate efficiency of the fungus Beauveria bassiana (Bals.) as a biological control agent in field applications against the European corn borer, Ostrinia nubilalis (Hub.) in corn fields (2008 season), the lowest concentration (2.5×10⁷) induced 26.87% reduction, while 5×10⁷ resulted 44.64% larval reduction, the highest concentration 1×10⁸ conidia/ml resulted 58.23 % larval reduction. Infested internodes  were reduced in values ranging between 23.78% and 66.53 % at different B. bassiana concentrations. The highest concentration (1×10⁸) reduced number of holes and number of broken tassels by 60.03 and 73.28 %, respectively. In 2009 season, reductions were 60.88, 62.63, 47.80 and 64.85 % for larvae, infested internodes, holes and broken tassels, respectively when the highest concentration (1×10⁸ conidia/ml) was applied. Treated rice plots (2008 season) by B. bassiana against the rice stem borer, Chilo agamemnon (Bles.) reduced dead hearts by 20.29, 43.59 and 55.0 % fifteen days post treatment. More reduction in dead hearts were obtained 30 days after treatment; 24.23, 50.17 and 71.88 % at 2.5×10⁷, 5×10⁷ and 1×10⁸ conidia/ml, respectively. Also, white heads were reduced, 30 days post treatment, by 34.87, 56.23 and 70.88 at 2.5×10⁷, 5×10⁷  and 1×10⁸ conidia/ml, respectively. In 2009 season, 30 days after applications, dead hearts were reduced by 26.48, 51.68 and 73.30, while white heads were reduced by 35.34, 62.26 and 72.04 % for 2.5×10⁷, 5×10⁷ and 1×10⁸ conidia/ml, respectively. Thus, two sprays by the fungus at two concentrations could be applied to get sufficient reduction in reducing the infestation by O. nubilalis and C. agamemnon in maize and rice fields.