This study was conducted to investigate the effects by supplementing concentrates during the late fattening phase in TMR (total mixed ration) feeding of Korean steer on the performance, carcass traits, physico-chemical characteristics and sensory test of the longissimus dorsi muscle. Eight Korean steers (24 months of age) averaging 604 kg in body weight were fed the TMR (Control) or the TMR supplemented with concentrates (TMR+conc.) until 30 months of age, then they were slaughtered. Steers fed the TMR supplemented with concentrates had a higher (p<0.05) intakes of dry matter and TDN (total digestible nutrient), carcass weight and marbling score compared to those fed TMR only, but carcass grade did not differ (p>0.05) between treatments. Amino acid concentration of the longissimus dorsi muscle did not differ between treatments, However, the highest concentrations were for glutamic acid and lowest for cystine; further, and the content of essential amino acids was highest for lysine, leucine, threonine, arginine, and isoleucine in that order. Cis-oleic acid and arachidonic acid of fatty acids in the longissimus dorsi muscle were higher (p<0.05) in the control condition compared to TMR+conc. The contents of oleic acids, palmitic acid, stearic acid constituted about 88% of the total fatty acids. Although the physico-chemical characteristics of the longissimus dorsi muscle did not significantly differ between treatments (p>0.05), but the sensory test results were lower for the TMR+conc. condition. These results suggest that supplementing concentrates during the late fattening phase of Korean steers resulted in increasing the carcass weight and marbling score. However, the cis-oleic acid content of the longissimus dorsi muscle was decreased as a result of supplementing concentrates, which could affect negatively in meat sensory evaluation.