Objective: To evaluate the insecticidal effect of diatomaceous earth (DE) against adults and nymphs of Blattella germanica. Methods: This cross sectional study has been done on the laboratory strain of German cockroaches. Two stages, nymph and adult, were exposed to six dose rates of the DE, 2.5, 5, 10, 15, 20 and 25 g/m2, at 24, 48 and 72 h exposure period. Mortality (number of dead cockroaches) was assessed after 24 h. Other exposed specimens were transferred to the beakers contained food and water for counting the retard mortality rate after 1 week. Results: Increasing in dose rates of DE increased mortality rate, so that the lowest and highest mortality rates were observed in 2.5 and 25 g/m2, respectively. The results of the statistical analysis showed no significant difference in the lethality of 50% of DE plus water on the German cockroach nymphs. Conclusions: Due to the resistance of German cockroach against organochloride, organophosphorus, carbamate and pyrethriodes insecticides, it is suggested to use DE for insect's control.