The cotton bollworm, Helicoverpa armigera (Lep.; Noctuidae) is one of the most important and most economic crop pests. This is a Polyphagous pest, and because of high mobility, high viability in adverse conditions, being multi-generated, and resistance to insecticides, it's very difficult to manage. The rate of Helicoverpa armigera (Lep; Noctuidae) damage in northern Iran has been reported in the normal years 10 to 25 percent and during the outbreak years of 50 to 70 percent. The use of new and safe pesticides has been very popular in recent years. In this study, nutritional effects of chitosan (an Iranian product) were studied under laboratory conditions on third and fifth instars of Helicoverpa armigera (Lep.; Noctuidae). This biopolymer was studied under laboratory conditions (27±2°C, 70±5% R.H. and L:D 16:8) at concentrations of 2.5, 4, 6, 8 and 10 g/kg of diet and three replicates. In the third instars, growth inhibition, after one day feeding of above concentrations was at least 20.56% and at most 37.34% and after two days feeding was at least 21.43% and at most 38.92%. The rates of growth stimulation also were at least 2.05% and a maximum of 22.14% after one day, and after two days feeding were at least 0.3% and up to 28.22%. Also, after two days of feeding from the concentrations studied, the inhibitory and stimulatory levels of feeding were varied from 0.0-0.15% and 20.96-35.1%, respectively. Generally, the effect of chitosan on third instars was growth inhibition and feeding stimulation. In the case of the fifth instars, growth inhibition after one day feeding from the above concentrations was at least 0.0% and at most 4.17%, and after two days feeding was at least 0.58% and 4.53%. The rate of growth stimulation after one day feeding was also at least 13.84% and at most 58.45%, and after two days feeding was at least 6.51% and at most 18.85%. Also, after two days of feeding from the concentrations studied, the inhibitory and stimulating levels of feeding varied from 0.14-6.28% and 5.89-29.19%, respectively. In general, the effect of chitosan on fifth instars was growth and feeding stimulation. The overall conclusion of comparing the effects of chitosan on larvae of the third and fifth instars indicates that the effect of chitosan, depending on the concentration and age of larvae, can be as growth inhibition or stimulation and nutrition inhibition or stimulation, which should be considered in the relevant studies.