This study aimed to evaluate the effects of essential oils (EOs) extracted from L. and Lam. on in vitro ruminal fermentation characteristics, selected rumen microbial populations, and methane production. GC-MS analyses allowed us to identify 89 compounds in both EOs. It was found that -β-caryophyllene predominated in (18.4%) and (24.1%). An in vitro (Ankom) test was performed to analyse the control and monensin groups, as well as the 50 µL or 100 µL EOs. The samples for volatile fatty acids (VFAs), lactate, and microbiological analysis were taken before incubation and after 6 and 24 h. The application of EOs of resulted in an increase in the total VFAs of acetate and propionate after 6 h of incubation. The applied EOs had a greater impact on the reduction in methane production after 6 h, but no apparent effect was noted after 24 h. Lower concentrations of and had a more pronounced effect on spp. and spp. than monensin. The presented findings suggest that and supplementation can modify ruminal fermentation, the concentrations of specific volatile fatty acids, and methane production.