Supplementation of β-mannanase, an enzyme that breaks down mannan, in a diet composed of high mannan contained feedstuffs (e.g., palm kernel meal, copra meal, soy hull; HMCF) improves utilization of feeds not only in monogastric animals, but also in goats and Holstein calves. No study, however, has been conducted to evaluate the effect of β-mannanase supplementation in Hanwoo (Bos taurus coreanae) growing heifers. Moreover, little is known whether β-mannanase supplementation also has beneficial effects in conventional corn-soy based diets (CS). The objective of this study was thus to investigate the effect of supplementation of β-mannanase in Hanwoo growing heifers fed a diet composed of either a CS or HMCF based concentrate mix. For this purpose, a feeding trial was conducted for 12 weeks using a total of 40 Hanwoo growing heifers (BW: 236.2 kg ± 1.1 kg, 11 mo of age). Total mixed rations (250 g/kg of ryegrass and 750 g/kg of concentrate mix) were offered ad libitum. The experimental design was 2 × 2 factorial approach with 2 different concentrate mixes (CS or HMCF based) and with or without a commercial feed β-mannanase (800,000 U/kg DM) product (CTCZYME, CTC Bio Inc., Seoul, Korea) at a level of 1 g/kg in concentrate mixes. In both diets, supplementation of β-mannanase significantly increased ADG of Hanwoo heifers by 95 g/d on average (111 g/d or 90 g/d for CS and HMCF, respectively; P < 0.05). No significant difference was observed in DMI among treatments (P > 0.10), and thus, β-mannanase supplementation tended to decrease feed conversion ratio (DMI/ADG) by 1.3 on average (P = 0.06). The ADG of CS (943 g/d) was significantly greater than that of HMCF (854 g/d), as expected (P < 0.05). These results indicated that supplementation of β-mannanase enhanced growth rate and feed efficiency of Hanwoo heifers regardless of the basal diets. Therefore, we conclude that supplementation of β-mannanase can be beneficial to improve the feed utilization in Hanwoo growing heifers.