In recent years, different formulations such as nanopowder can serve for the development of essential oils (EOs) and their compound derivatives with enhanced activity or environmental friendliness. The cowpea seed beetle, Callosobruchus maculatus F. is a major pest of cereals and legumes. This insect causes quantitative and qualitative losses due to perforation damage, lowered market value and poor viability of seeds. In this study, nanopowder (NP) from Mentha longifolia L. (Lamiaceae) oil has been produced and its effect in comparison with non-formulated EO on adult stage (1-3 days) of C. maculatus F. has been analyzed. The experiments were conducted at controlled experimental conditions (27±2 °C, 65±5% relative humidity and darkness). Chemical composition of M. longifolia oil was also studied by using Gas chromatography-mass spectrometry (GC-MS) apparatus. The main constituents were pulegone (28.84%), 1,8-cineol (19.6%), p-menthan-3-one-cis (8.2%), β-pinene (6.46%) and p-menthan-3-one-trans (5.86%). The results showed that the fumigant toxicity of EO in first 24 hours after being experiment was higher compared with NP. The LC50 values of EO and NP were 10.1 and 46.6 µl/l, respectively. However, in the long term the EO's effect decreased and NP's effect increase. At 25 µl/l (lethal concentration for 80% mortality), The LT50 values calculated from EO and NP were estimated about 2.2 and 37.9 days, respectively. The durability of NP lasted for 51 days. Therefore, this technique can maintain the toxicity properties of EO in the long term. The nanopowder slow-release formulation may represent a new model of biopesticides and this should be considered in the organic agriculture.