Four ruminallycannulated Holstein steers (540 ± 35 kg initial body weight) were used in a 4 × 4 Latin square design over 21-day periods to examine effects of supplementing a basal diet (CON) with thyme oil (THY, 500 mg/kg DM) or cinnamon oil (CIN, 500 mg/kg DM) on nutrient digestibility, ruminal fermentation characteristics and rumen microbial populations. Monensin (MON, 33 mg/kg DM) was used as positive control. Steers were fed a basal diet as total mixed ration (TMR) ad libitum. The results indicated that dry matter intake (DMI) and apparent digestibility of nutrients were not affected by additives. Dietary supplementations did not affect ruminal pH and ammonia nitrogen concentration. Total volatile fatty acid (VFA) concentrations and proportions of acetate and butyrate were not affected by essential oils (EO), but there was a trend (P