Tribolium castaneum is a secondary destructive insect which causes significant damage to grains. In this study we evaluate the insecticidal activity of diatomaceous earth (DE) and malathion against this insect and its offspring. Also, the morphological effects of these materials on adult and larvae were studied using scanning electron microscopy (SEM). The results showed that malathion had higher toxicity than diatomaceous earth by comparing their LC₅₀ values over exposure periods. DE and malathion significantly reduced the progeny production of the target insect by 2.4 to 95.8 % and 12.6 to 57.4 % for malathion and DE, respectively. SEM analysis revealed external morphological alterations. Firstly, adult treated with DE had particles stuck to the mouth parts and abdominal end, and the sensilla of the treated area was damaged. In addition, the abdominal end (ovipositor tip) of both treatments appears enlarged and prominent. Secondly, for larvae DE and malathion treatments make (maxillary palp, labial palp) of the mouth parts and the abdominal end in shrinking shape. Also, spiracles and thorax segments appear shrunken and losing of spiracles hairs. Overall, DE causes physical damage observable by SEM. DE is a promising alternative to pesticides due to its insecticidal activity as well as its relative safety to human and environment so, it can be used as natural pesticides in stored product pest control.