The objective of this study was to determine the effect of inclusion of essential oil thymol on the incubation on gas production kinetics, volatile fatty acids (VFA), organic matter digestibility (OMD) and metabolizable energy (ME) contents of alfalfa hay. Gas productions were determined at 0, 3, 6, 12, 24, 48, 72 and 96 h incubation times. Thymol were added in the ratio of 0, 50, 100 and 200 mg/L. Gas production kinetics were determined using the equation Y = A (1-exp-ct). The thymol addition had a significant effect on the gas production kinetics, OMD and ME of alfalfa hay. Thymol at 200 mg/L resulted in 22.77% of decrease in potential gas production (A). The mean decrease in potential gas production per mg thymol supplementation was 0.0836 ml. The mean decreases in ME and OMD per mg thymol supplementation were 0.0132 (ME unit) and 0.086 (digestibility unit) respectively. The mean decreases in truly digestible dry matter (TDDM) and neutral detergent fibre (NDFD) per mg thymol supplementation were 0.0546 and 0.0748 digestibility units respectively (P < 0.05; P < 0.001). As a conclusion, thymol exhibit significant anti-microbial activity causing an inhibition of the overall fermentation process.