An in vivo experiment was performed to evaluate the effects of calcium ammonium nitrate (CAN; 5Ca (NO3)2·NH4NO3·10H2O, 75% DM NO₃⁻) supplementation on enteric CH4 emissions, ruminal fermentation and microbial community and productivity in Hanwoo (Korean native cattle) steers. Twenty Hanwoo steers (average body weight 348 ± 18 kg) were randomly allocated to 5 pens (4 head/pen) balanced for body weight, and then randomly assigned to 1 of 2 dietary treatments (2 head/treatment/pen): (1) the control (CON), 500 g/kg long stem timothy hay and 500 g/kg commercial concentrates; (2) NIT, control diet top-dressed with 2.5% CAN (1.578% NO3- in the dietary DM) for 115 d in a completely randomized design. Methane measurement was performed for 3 d starting from days 21, 51, 80, and 112 using whole-body respiratory chambers (RC). Rumen fluid and blood samples were collected on days 25 and 26, respectively. A decrease (P = 0.05) in dry matter intake (DMI) was observed in steers fed NIT both in the pen and RC. However, average daily gain and gain:feed were not adversely affected. Additionally, methemoglobin concentration was not influenced by the treatment, suggesting no potential toxicity of NO₃⁻ in steers fed NIT. A decrease (P < 0.05) in CH4 production (g/d) was noted in NIT throughout the feeding trial, but CH4 yield (g/kg DMI) was not different (P > 0.05) between treatments. Ruminal pH, ammonia concentration, and total volatile fatty acid production were not affected by the treatment, but NIT increased and decreased (P < 0.01) acetate and butyrate proportions, respectively, compared with CON. Steers in NIT group had increased (P < 0.05) ruminal abundance of Ruminococcus sp. and a tendency for increased Succiniclasticum (P = 0.07), and Selenomonas ruminantium (P = 0.07). However, NIT decreased (P < 0.05) the abundance of Olsenella, Atopobium and tended to decrease (P = 0.07) Methanobrevibacter ruminantium in the rumen. Overall, dietary supplementation of NO₃⁻ demonstrated a potential to reduce enteric CH4 production (g/d) by an average of 25.4% in Hanwoo beef cattle over time without causing NO₃⁻ intoxication in the early stage or negatively affecting growth performance throughout the trial. However, the lack of a sustained mitigation effect on CH₄ yield (g/kg DMI) suggests that further evaluation of NO₃⁻ supplementation under different dietary conditions is necessary.