There mainly exists four major myosin heavy chains (MyHC)(ie, I, IIa, IIx, and IIb) in growing pigs. The current study aimed to explore the effects of low-protein diets supplemented with varying branched-chain amino acids (BCAAs) on muscle fiber characteristics and the AMPK-SIRT1-PGC-1α axis in skeletal muscles. Forty growing pigs (9.85�0.35 kg) were allotted to 5 groups and fed with diets supplemented with varying leucine: isoleucine: valine ratios: 1: 0.51: 0.63 (20% crude protein, CP), 1: 1: 1 (17% CP), 1: 0.75: 0.75 (17% CP), 1: 0.51: 0.63 (17% CP), and 1: 0.25: 0.25 (17% CP), respectively. The skeletal muscles of different muscle fiber composition, that is, longissimus dorsi muscle (LM, a fast-twitch glycolytic muscle), biceps femoris muscle (BM, a mixed slow-and fast-twitch oxido-glycolytic muscle), and psoas major muscle (PM, a slow-twitch oxidative muscle) were collected and analyzed. Results showed that relative to the control group (1: 0.51: 0.63, 20% CP), the low-protein diets with the leucine: isoleucine: valine ratio ranging from 1: 0.75: 0.75 to 1: 0.25: 0.25 especially augmented the mRNA and protein abundance of MyHC I fibers in BM and lowered the mRNA abundance of MyHC IIb particularly in LM (P< 0.05), with a concurrent increase in the activation of AMPK and the mRNA abundance of SIRT and PGC-1α in BM (P< 0.05). The results reveal that low-protein diets supplemented with optimal BCAA ratio, ie 1: 0.75: 0.75-1: 0.25: 0.25, induce muscle more oxidative especially in oxido-glycolytic skeletal muscle of growing pigs. These effects are likely associated with the activation of the AMPK-SIRT1-PGC-1α axis.