High energy diets have been used for enhancing marbling fat in high quality beef cattle. However, this has been associated with reduced yield grade when the steers were fed for long-term. The aim of this experiment was to determine the effect of additional 3% TDN (77%) diet compared with control diet (74%) on the level of adipogenic and myogenic gene expressions at 3 different endpoints. A 2 × 3 factorial arrangement (High, Control, and 26, 28, 30 mo endpoints) was used to feed 48 Hanwoo steers. Four steers were fed in same pen and 12 pens were used for treatment. Longissimus dorsi (L.D.) muscle was collected within 10 min of harvest for analysis of PPAR?, SCD, GLUT4, MHC1, MHC2X, and myogenin mRNA abundance. Real-Time RT-PCR was used to measure the quantity of respective mRNA relative to a ribosomal protein subunit 9 (RPS9) mRNA. Data were analyzed as a completely randomized design using the MIXED model, each treatment performed in triplicate. Difference between control and treatments were determined using the LSD procedure. Overall ADG did not differ between high-energy diet or control diet (P > 0.05). ADG tend to increase in high-energy diet at 30 mo endpoint compared with control diet (P = 0.09). Marbling score and carcass weight were greater at 30-mo endpoint than at other endpoints (P < 0.05). Yield grade was not different among endpoints (P > 0.05). Percentage of protein in L.D. muscle tend to decrease at 30 mo endpoints (P = 0.09). Real-time quantitative PCR revealed that the mRNA content of MHC2X in muscle from high-energy diet cattle decreased (diet x endpoint, P < 0.05) as compared with the control group. There was no interaction for MHC1 gene expression among 3 endpoints (P > 0.05). These data indicated that high-energy diet decreased relative mRNA level of MHC2X on long-fattening periods. These fast-glycolytic genes may affect muscle protein composition in Hanwoo L.D. muscle.