Abstract: Abstract: The present study aimed to investigate the influence of dietary supplementation with lactobacillus on muscle fiber properties and meat quality in Sunit sheep. Totally 12 Sunit sheep aged 3 months with good body condition were selected, and then two groups were randomly divided. The control group was fed a basal diet (a typical corn-soybean diet) without any antibiotics, drugs, or growth promoters. Lactobacillus group was supplemented with lactobacillus plantarum at 3×1010 cfu/g based on the diet of the control group, where the feeding period lasted for 90 days. After slaughter, the longissimus dorsi muscle was taken for subsequent test. An ATPase histochemical staining and a real-time fluorescence quantitative technique were utilized to determine the muscle fiber properties, the mRNA expression of myosin heavy chains (MyHCs) and genes related to mitochondrial biogenesis. In addition, the activity of metabolic enzymes, growth performance, and meat quality were measured to explore the relationship between muscle fiber properties and meat quality. The results showed that the dietary supplementation with lactobacillus significantly increased the value of pH24 (P<0.01), while reduced the value of b* (P<0.05) and cooking loss (P<0.01) of meat. There was no significant effect on the growth performance and other meat quality indicators (P>0.05). The analysis of muscle fiber showed that the dietary supplementation with the lactobacillus increased the number ratio of type IIA muscle fibers (P<0.01), but decreased the number ratio of type IIB muscle fibers (P<0.01), where there was no significant effect on the diameter and cross-sectional area of muscle fibers. At the same time, MyHCⅠ (P<0.05), MyHCⅡa (P<0.01), and MyHCⅡx (P<0.01) mRNA expression were significantly higher than those of the control group. The enzyme activity of Succinate Dehydrogenase (SDH) in the lactobacillus group was significantly higher than that in the control group (P<0.05), whereas, the enzyme activity of Lactic Dehydrogenase (LDH) became significantly lower (P<0.05). In addition, the dietary supplementation with the lactobacillus increased the mRNA expression of AMP-activated protein kinase α1 (AMPKα1) (P<0.01), Sirtuin1 (SIRT1) (P<0.01), and Cytochrome c oxidaseⅣ (COXⅣ) (P<0.05) in the longissimus dorsi muscle of Sunit sheep. There was no significant difference (P>0.05) in the mRNA expression levels of AMP-activated protein kinase α2 (AMPKα2), Peroxisome proliferator-activated receptor gamma co-activator 1-alpha (PGC-1α), and Mitochondrial transcription factor A (TFAM) in the control group, particularly similar to that of lactobacillus group. Consequently, the lactobacillus increased the oxidative metabolism of muscle, thereby promoting the transformation of muscle fiber types to more oxidation type of longissimus dorsi muscle in Sunit sheep, indicating much better meat quality than before. In any way, the dietary supplementation with lactobacillus significantly promoted the generation of more oxidative muscle types, which can be associated with the AMPKα1- SIRT1- PGC-1α axis and mitochondrial biogenesis.