Abstract: This study was performed to evaluate the effect of different cereal source in choice feeding systems on performance and on emission of carbon dioxide (CO2) and enteric methane (CH4) in dairy Awassi ewes. Total 16 dairy ewes were divided into two groups: the corn based free choice (CFC) group received feed ingredients separately (corn, wheat bran, soybean meal (SBM), cottonseed meal (CSM) and alfalfa hay) and the barley based free choice group (BFC) group received barley instead of corn as carbon hydrate source. The results showed no significant differences (p > 0.05) between treatments in live weight, live weight gain, milk yield, milk composition and CO2 production. However, the results of CH4 measurement indicated significant differences between groups in the amounts of CH4 produced. The ewes in the CFC group produced less CH4 than the ewes that received the BFC system (CFC: 21.82; BFC: 38.34 g/day/sheep). The results indicate that the CFC system modified ruminal fermentation and affected the Volatile Fatty Acid (VFA) components and levels in ruminal fluid. In ewes on the CFC system, the level of propionate was greatly increased (CFC: 19.77 vs. BFC: 14.53%) and the level of acetate decreased (CFC: 68.34 vs. BFC: 75.58%). Butyrate level was not changed relative to the total VFA components. There were no significant differences in ruminal pH level between treatments. The results indicate that the CFC system has a potential mitigating effect on enteric emission of CH4 but not CO2.