Present study was conducted to investigate the effect of including plant essential oils on invitro ruminal fermentation and microbial nitrogen synthesis of a dairy cow diet rich in concentrate. The treatments consisted of the diet alone (control; BD) as well as containing 50 and 100 μl L-1 essential oil of thyme (BDT), mint (BDM), savory (BDS), or a mixture of the essential oils at the rate of 1:1:1 (BDmix). Essential oilsdecreased gas production at 24, 48 and 96 h of incubation compared with that of BD. However, mint at the rate of 50 or 100 μl L-1 resulted an increase in the microbial nitrogen when compared toBD, BDS andBDT. The nitrogen content of truly undegradedresidu (NDFN) content and NH3-N concentration were lower,while the dry matter digestibility was greater in the BDmix, regardless of dosage levels, as compared with the control. The inclusion of a mixture of essential oils at 50 μl L-1 to the basal diet caused intensified dry matter disappearance,in comparison to other treatments. Results showed that the synergetic effects of essential oils together in a dairy cow diet of rich in concentrate can alter rumen microbial fermentation and improve microbial protein yield.

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