In ruminants, the rumen provides the nutrients needed for animal physiological requirements and for facing ambient environmental effects. Therefore, any rumen manipulation affects animal biological functions. The goal of the present study is to investigate the effects of different levels of zinc nanoparticles (nano-ZinO) on in vitro ruminal feed degradation, ruminal fermentation, and total gas production. The fig leaf extract was used as a reducing agent for zinc ions to prepare nano-ZnOs instead of chemical reducing agents (green chemistry). The X-ray diffraction (XRD) pattern, scanning electron microscope (SEM), transmission electron microscope (TEM), Fourier transform infrared (FT-IR) spectroscopy, and dynamic light scattering (DLS) were used to evaluate the nano-ZnOs. The batch culture technique was used to evaluate the impact of supplemented diets with different levels of nano-ZnO; 30, 60, 90, 120, 150 and 180 mg/Kg dry matter (DM) diet on in vitro rumen fermentation characteristics. The findings demonstrated that fig leaf extract has the capacity to produce spherical zinc oxide nanoparticles with an average size of 25 nm. The crystallographic structure of the green synthesized nano-ZnO was confirmed using XRD analysis. The DLS showed that the manufactured nano-ZnO particles had an average diameter of roughly 215 nm. The diet supplemented with 30 mg nano zinc/kg DM increased the degradability values (P<0.05) of the dry matter (DM), organic matter (OM), neutral detergent fibre (NDF), acid detergent fibre (ADF), and rumen fermentation parameters, including total gas production (TGP), lactic, acetic, propionic, and butyric acids, and ammonia-nitrogen (NH3-N) concentrations to the maximum compared to the control. The increased level of nano-ZnO supplementation gradually led to a decline in rumen fermentation and diet degradability. It could be concluded that the supplemented diet with 30 mg nano-ZnO/kg DM is the optimal level for the utilization to guarantee the highest feed nutrient digestion and enhancement of fermentation efficiency in the rumen.