Essential oils, such as cinnamaldehyde, are secondary metabolites obtained from plants that appear to be natural alternatives to antibiotics and function similarly to ionophores. The objective of this study was to evaluate the effect of the essential oil, cinnamaldehyde, on feed intake, rumen fermentation, nutrient digestibility, and milk components in lactating dairy cows. Six lactating Holstein dairy cows (3 ruminally cannulated and 3 noncannulated) averaging (mean ± SD) 263 ± 41 d in milk (DIM) and 754 ± 45 kg of body weight (BW) at the beginning of the study were selected. Cows were randomly assigned to 1 of 3 treatments in a replicated 3 × 3 Latin square design with 19 d periods (14 d for diet adaptation and 5 d for sample collection). Treatments were 0, 2, or 4 mg/kg of BW of the essential oil, cinnamaldehyde. Cinnamaldehyde was mixed with 40 g of corn meal and top-dressed onto the total mixed ration (TMR). Diet was fed as a TMR and contained 37% corn silage, 18.5% mixed-mostly grass silage, 24.5% energy mix, 16.5% protein mix, and 3.5% vitamin and mineral mix on a dry mater (DM) basis. The dietary nutrient composition averaged 15.1% crude protein (CP), 37.8% neutral detergent fiber (NDF), and 24.7% acid detergent fiber (ADF), for all 3 periods. Cows were fed and milked twice daily. No significant differences were observed for dry matter intake (DMI; mean = 24.6 kg/d), milk yield (mean = 28.4 kg/d), 3.5% fat-corrected milk (FCM; mean = 30.6 kg/d), and 3.5% energy-corrected milk (ECM; mean = 30.7 kg/d). Dose of cinnamaldehyde did not have any effect on milk components or rumen fermentation and pH (P > 0.05). There were no differences in nutrient digestibility, but there was a trend for a quadratic effect for DM digestibility: 74.4%, 76.3%, and 73.7% for treatments 0, 2, and 4 mg/kg of BW of cinnamaldehyde, respectively (P = 0.09). Overall, supplementing lactating dairy cows with the essential oil, cinnamaldehyde had no effect on feed intake, rumen fermentation, nutrient digestibility, or milk components.