This experiment was conducted to study the effects of different xylooligosaccharide (XOS) supplemental levels on growth performance, longissimus dorsi muscle nutrient content and muscle fiber type composition of piglets, in order to determine the optimal dose of XOS supplemental level and the possible regulatory mechanisms of XOS for muscle nutrient and muscle fiber type composition. A total of 120 healthy weaned Duroc×Large White×Landrace piglets with 21 days of age were randomly assigned into 4 groups with 6 replicates per group and 5 piglets per replicate. Pigs in the control group were fed a basal diet and the others in the experimental groups (groups Ⅰ, Ⅱ and Ⅲ) were fed the basal diets supplemented with 100, 250 and 500 mg/kg XOS, respectively. The experiment lasted for 56 days. When the experiment was over, one pig per replicate close to the average weight was chosen to slaughter sampling. The results showed as follows: 1) compared with the control group, average daily gain and average daily feed intake (ADFI) of pigs in group Ⅰ were significantly increased (P<0.05), and ADFI of pigs in groups Ⅱ and Ⅲ was significantly increased (P<0.05), too. But there were no significant differences in the final weight and the ratio of feed to gain among all groups (P>0.05). 2) There were no significant differences in the contents of crude protein, ether extract, free amino acids such as leucine, glycine and aspartic acid among all groups (P>0.05), while the other amino acids in experimental groups rose or fell with different levels compared with the control group. 3) The myosin heavy chain (MyHC) Ⅱa and MyHC Ⅱx mRNA relative expression levels in group Ⅰ were significantly higher than those in the control group (P<0.05), and MyHC Ⅰ mRNA relative expression level in group Ⅱ was significantly increased compared with group Ⅲ (P<0.05). Compared with the control group, the mRNA relative expression levels of AMP-activated protein kinase alpha, silent mating type information regulation 2 homolog 1 and uncoupling protein 3 were significantly increased (P<0.05), and there were no significant differences in the mRNA relative expression level of peroxisome proliferator-activated receptor-γ coactivator-1α among all groups (P>0.05). In conclusion, the suitable XOS supplementation in piglet diet can improve the growth performance and promote the mRNA relative expression levels of genes related to slow-twitch muscle fiber, and the recommend supplemental level is 100 to 250 mg/kg under the present experiment.