An in-vitro study was carried out to investigate the effect of using different levels of the essential oils of Marjoram (Origanum majorana L.) or Basil (Ocimum basilicum L.) as a natural feed additive on ruminal fermentation, total gas production, ammonia concentration, dry matter and cell wall contents (NDF and ADF) digestibility.Eleven experimental groups were used by in-vitro batch culture technique. The basal diet consisted of a total mixed ration (TMR) in all treatments, the experimental treatments were: 50% CFM, 50% alfalfa hay (control), control diet plus 5, 10, 15, 20 and 25 ml of Marjoram oil or Basil oils / kg DM for T1, T2, T3, T4, T5, T6, T7, T8, T9 and T10, respectively. No differences (p>0.05) were observed between Marjoram or Basil oils in pH, NDFd and ADFd at all different levels compared with the control diet. Ammonia concentrations showed significant (p < 0.05) decreased for all the experimental diets compared with the control diet, except T6. Also, short chain fatty acid (SCFAs) contents were decreased for all treatments compared with control, except T8 and T9. The values cleared that Marjoram oils supplementation to diets showed a significant decrease (p < 0.05) in total gas production versus control which recorded the highest value (128 ml) except T8 and T9.  There was a significant increase (p < 0.05) in dry matter degradability (DMd) between T4 and T9 compared with the control. Cell wall contents (NDF and ADF) digestibility values were not significant affected by oils supplementations. It could be concluded that supplementation of Marjoram or Basil oils to diets had negative influence on ruminal fermentation parameters (SCFAs, ammonia and total gas production). The Marjoram oil showed significantly superior DM degradability and reduction total gas production compared to the Basil oil.