Different formulations of pepper mint oil, i.e. bulk, nano-emulsion and nano-encapsulation were evaluated against 2nd and 4th instar larvae of the cotton leafworm Spodoptera littoralis (Boisd.). Their effects on enzymatic activities and bionomics were also investigated. Transmission Electron Microscopy (TEM) showed that the mean particle size of the prepared nano-emulsion ranged between 20-90nm. The encapsulation efficiency and loaded capacity percentages exhibited that distilled H2O was more suitable than ethanol in preparation of nano-encapsulation particles. Bioassay treatments showed LC50's on 2nd instars larvae were 70.59, 12.23 and 21.72 ppm for pepper mint oil, pepper mint nano-emulsion and loaded nanoemulsion, respectively. While on 4th instars these values were 80.47, 26.14 and 67.69 ppm, respectively. LC90's on 2nd instars were 104.75, 33.97 and 30.14 for pepper mint oil, pepper mint nanoemulsion and loaded nano-emulsion, respectively. While on 4th instars these values were 177.66, 71.37 and 102.29 ppm. Toxicity index and relative potency proved that nano-emulsion was more effective than loaded nano-emulsion, while the bulk preparation was the least effective one. Results of enzymatic activities showed marked effects of the three formulations of mint oil, significant inhibitions  were recorded for amylase, invertase, trehalase, protease and alkaline phosphatase, while significant increase in activities of cuticle phenoloxidase and chitinase. Effects of adding the three formulations to artificial diet of 2nd instar showed significant effects of the insect bionomics. Larvae durations, percentage mortalities, were increased as well as larval malformations. Also, pupal duration, percentage pupal mortality and pupal malformation were increased, while pupal weight was decreased. Adults longevity showed insignificant effects, while female fecundity and egg % fertility showed significant responce.