Beauveria bassiana is one of Oryzaephilus surinamensis entomopathogenic fungus. In this study the sub lethal doses effects of the pathogen on saw tooth beetles' growth rate, nutrition ability and weight were investigated in terms of nutrition three Dates cultivars including Sayer, Zahedi and Deiri long in vitro condition. Adults were treated by 5 sub lethal doses of including 102, 5×102, 103, 5×103 and 104 and compared by control. Then, the sub lethal doses of reduced weight of each cultivar were calculated by using Log-Probit analysis method. The tested doses of pathogen had decreasing effects on growth rate indeces. The synergist of fungal pathogen and cultivars on reduce growth rate were gradually by the increasing pathogen doses. Minimum and maximum sub lethal doses for reducing the 50 percent of weight were showed in population grew on Sayer and Zhedi cultivars and equivalent to 3.99×102 and 9.01×102 spores/ml respectively. Minimum and maximum sub lethal doses for reducing the 50 percent of nutrition ability were showed in population grew on Deiri and Sayer cultivars and equivalent to 2.31×102 and 8.37×102 spores/ml respectively.The results of this study showed that the fungus B. bassiana had a significant effect in reducing the growth and nutrition potentials. The pathogen ability to reduce the growth and nutrition potentials was in addition depended on the fungi dose and Date cultivars that feeding by the pest. Significant differences were observed in all concentrations in the control group. All concentrations have a decreasing effect on MRGR and gradually increased with the increase in synergistic effects of reducing the growth of saw tooth beetles' growth.