A field study was conducted in the location of the department of Horticulture – College of Agriculture to determine the effect of Beauveria bassiana against the two spotted spider mites Tetranychus urticae on potato. The concentrations of B.bassiana used were 1x104, 1x106, 1x108, in addition to the positive control as the pesticide fytoclean and negative control as distilled water. The best response was found at 1x108 of the fungus. The number of mite eggs was reduced from 42.33 in the control to 4.33, 4.67, 3.33 egg/leaf for the three concentration. respectively. Moreover, the mobile stages were also reduced from 26.00 in control to 6.33, 3.67, 2.67 individual/leaf respectively. However, the insecticide fytoclean used as positive control reduced the number of eggs and moving stage to 1.67, and 1.00 individual/leaf. Results of this study indicate the importance of introducing the fungus Beauveria bassiana in the biological control of mites.