---

The aim of this research was to study the effect of iron oxide nanoparticles (FeO NPs) on the growth, differentiation, anatomy, and physiology of pepper (Capsicum annuum L.) on the basis of a completely randomized design in vitro condition. Seedlings were cultured in MS medium containing four concentrations of FeO NPs (0, 1, 10, and 20 mgl^-1). Also, the effect of the different concentrations of FeO NPs on callus formation under two various hormone conditions (0.5mgl^-1 2,4D+0.5 mgl^-1 BAP or 0.5 mgl^-1 BAP+1 mgl^-1 Kin) were assessed. The results showed that the application of FeO NPs significantly increased biomass accumulation in both roots and shoots. Moreover, FeO NPs enhanced the concentrations of photosynthesis pigments (chllrophyll a, chlorophyll b, and carotenoids). The presence of FeO NPs in culture medium affected callus formation in a hormone-dependent manner. Different concentration of FeO NPs induced the callus formation under 2, 4-D and BAP treatments. However, it did not significantly increase callus formation under the kinitin and 2,4-D. The findings of this research indicated that the application of FeO NPs at optimized doses may improve plant production, especially in vitro condition.