The aim of the current study was to evaluate the effect of increasing doses of essential orange oil in vitro gas production, volatile fatty acid (VFA), methane, ammonia production, metabolisable energy (ME), organic matter digestibility (OMD), true dry matter digestibility (TDMD) and neutral detergent fiber digestibility (NDFD). Gas productions ofsoybean meal were measured at 0, 3, 6, 12, 24, 48, 72 and 96 h incubation times in the presence (100, 200, 400, 800 and 1200 mg/L) and in the absence of essential orange oil. The gas production kinetics were estimated using the non-linear exponential equation y = A (1-exp-ct). The supplementation of essential orange oil significantly (P<0.001) decreased fermentation parameters and estimated parameters such as the potential gas production (A), ME, TDMD, OMD, NDFD, total VFA, methane and ammonia productions. The mean reductions in A, ME, TDMD, NDFD, OMD, total VFA, methane and ammonia productions were 0.022, 0.033, 0.00209, 0.0197, 0.0213, 0.0414, 0.0207, 0.0093 units per mg essential orange oil supplementation respectively. The current study revealed that essential orange oil had significant anti-microbial activity causing an inhibition of the overall fermentation process. Therefore, before large scale implementation, further investigations are required to determine the effect of essential orange oil on voluntary food intake, animal performance and the profitability of the supplementation in vivo.