The use of mineral nanoparticles as replacer for traditional formulas of mineral dietary supplements for ruminants is a new approach. The main aim of this study was to evaluate effects of dietary Cr nanoparticles (Cr2O3-NPs) supplementation at different levels (0.3, 0.6, 0.9, 1.2, 2.1 and 3 mg//Kg DM) on feed fermentation and degradation in the rumen using the in vitro batch culture technique. Diet supplementation with Cr2O3-NPs at 0.3 mg/kg DM showed the highest (P<0.05) values of total gas production (TGP), lactic acid, acetic acid, propionic acid, butyric acid and ammonia-nitrogen (NH3-N) concentrations, dry matter (DM), organic matter (OM), neutral detergent fiber (NDF) and acid detergent fiber (ADF) degradation. Beyond 0.3 mg/kg of Cr2O3-NPs supplementation level, there is an inverse relationship between the dietary level of Cr2O3-NPs supplementation and the percentages of DM, OM, NDF and ADF degradation, TGP volume and NH3-N concentration. By comparison with the control, no negative effect of Cr2O3-NPs supplementation on all ruminal fermentation parameters up to 1.2 mg/kg DM. Supplementation of Cr2O3-NPs up to 3mg/kg DM has no negative impact on concentrations of lactic acid, propionic acid and butyric acid, but significantly decreased the TGP volume, acetic acid NH3-N concentrations and A/P ratio when compared to the control. It can be concluded that dietary Cr supplementation in nanometric form had a positive effect on ruminal fermentation traits and improving nutrients degradation rates in vitro. The optimum level of nano-Cr addition to the ruminants' diet was at 0.3mg/kg DM under the current trial conditions.