This study was conducted to investigate the effects of supplementation of direct-fed microbials (DFMs) during the growing and finishing periods on growth performance and carcass characteristics in Hanwoo steers. DFMs were administered only during the first 3 months after arrival and the final 3 months before slaughter, and the feeding trial lasted for 740 days. During the growing phase, the treatment group showed a significantly higher average daily gain (ADG) of 0.89 kg compared to 0.80 kg in the control group (p<0.05). No significant differences were found between groups during the fattening phase. In the finishing phase, ADG was significantly higher in the treatment group (0.59 kg vs. 0.53 kg; p<0.05), although concentrate intake increased by 6.17% without statistical significance. At the end of the entire feeding period, the treatment group had a higher final body weight (803.05 kg vs. 763.51 kg), and overall ADG was significantly greater (p<0.05). For carcass traits, cold carcass weight was significantly higher in the treatment group (p<0.05), while no significant differences were observed in backfat thickness, loin eye area, or marbling score. However, the proportion of carcasses graded 1++ was higher in the treatment group (50.0%) than in the control group (30.0%). In conclusion, supplementation of direct-fed microbials during the growing and finishing periods improved average daily gain and increased the proportion of carcasses with higher quality grades in Hanwoo steers.