The objective of this study was to evaluate in vitro and in vivo potential of a specific commercial blend of essential oil active compounds (BEO) to improve rumen fermentation and general dairy cow performance. First, the main active components of the BEO were identified and quantified. Then, the in vitro experiment was conducted using 24 h batch incubation of buffered rumen fluid. The BEO was added at 0, 20, 100, 200, 600, and 1000 mg/l of culture fluid, with substrate containing forage and concentrate at a ratio of 60:40 dry matter (DM) basis. The concentrations of 600 and 1000 mg/l decreased (P < 0.05) methane production per dry matter incubated (DMi) by 5.7% and 17.1%, respectively, and a concentration of 1000 mg/l decreased ammonia-N concentration by 10.0%. However, these reductions were accompanied by a decrease (P < 0.05) in apparent dry matter disappearance (aDMd), and lowered (P < 0.05) the net production of volatile fatty acids (nVFA) indicating, that there were no beneficial selective inhibitory properties of BEO supplementation. For the in vivo experiment, 30 lactating Holstein cows (two primiparous and 28 multiparous) in mid-lactation were randomly assigned to two treatments. During a 15 week period, cows in the control group (CON; n=15) were fed the basal diet with no additive, and cows in the other group (BEO; n=15) received the same diet supplemented with 1.2 g/cow/ d BEO. Addition of BEO did not affect (P=0.218) DM intake and milk yield (P=0.102). For feed efficiency (P=0.047) and body weight (P=0.014), the treatment×time interactions were observed, with cows fed BEO having a lower average feed efficiency and a higher average body weight. BEO treatment tended to decrease the proportion of milk fat (P=0.072), without affecting other milk quality parameters. BEO supplemented cows had lower VFA concentrations in rumen fluid (P=0.006), and correspondingly, pH values were higher (P=0.018). No differences were detected between the treatments for proportions of individual VFA and ammonia-N concentration. The responses of dairy cows on BEO were more pronounced after the 5th week of supplementation. This indicates the necessity of a longer adaptation period than commonly used in studies evaluating the effects of essential oils (EO). Due to energetic shift toward body weight gain additional attention should be paid to the systemic effects of EO. Generally, results from this study suggest that mid-lactation dairy cows did not benefit from being supplemented with BEO.