The experiments were carried out at three locations in Gharbia governorate which heavily infested with corn borers larvae, Ostrinia nubilalis and Sesamia cretica, on maize stalks abandoned in the field, these locations were Tanta, El-Santa and Quator locations during the period from November to April in 2014/15 season. A proportion reduction in number of the two pests occurred during overwintering in maize stalks by entomopathogenic fungi, Beauveria bassiana isolates. The rate of reduction revealed 28.57, 6.74 and 10.41 % of O. nubilalis larvae at Tanta, El-Santa and Qutour, resepectively and 7.69 % in S. cretica larvae only in El-Santa. Seventeen fungi isolates of the fungus B. bassiana were found on O. nubilalis larvae at the three locations then the germination of fungi isolates was measured and found that the Bb-OLT 26 Jan isolate was the highest of  conidia germinated then selected to bioassy study. The two fungi isolates Bb-OLT 26 and Bb-SLE 26 Jan were treated on O. nubilalis  and S. cretica, respectively in labortatory conditions with concentrations of 10⁴, 10⁵, 10⁶, 10⁷ and 10⁸ conidia/ml. Therefore the LC₅₀ values were 1.23 x 10⁵and 1.18 x 10⁷ conidia/ml. for Bb-OLT 26 and Bb-SLE 26 Jan isolates, respectively. This results indicated that the entomopathogenic fungus, B. bassiana is an effective a natural pathogen causing a consideral mortality in the hibernated larvae of O. nubilalis which may be important in the natural regulation of the pest and encourage to apply it on the field populations at maize fields.