Entomopathogenic fungus Beauveria bassiana was tested as biological control agent against forest nursery pests Poecilocerus pictus, and Brachutrypes portentosus in the laboratory at the Pakistan Forest Institute, Peshawar under the controlled temperature of 25-35°C and 55-65% relative humidity, for evolving their best possible biological control. B. bassiana fungus was tested in the doses of 0.8 x 104 spores/ml, 0.6 x 104 spores/ml in the study. The data showed that B. bassiana has given a non- significant result against the pests even after 72 hours but results were significant after 96 hours upto 168 hours of treatment. The study showed that after treatment the fungus first established and then caused the mortality of Poecilocerus pictus within 2-3 days while in case of Brachutypes portentosus within 7 days. The fungal hyphae developed in the treated insect pests first in their soft body parts like thorax and abdomen covering the whole body within a period of two weeks. The study showed the susceptibility of these insects to the fungus B. bassiana which could be used as biological control agent in the field.