This study was designed to evaluate the usage of Bacillus subtilis fmbj (BS fmbj) in broiler diets, and its effects on the antioxidant capacity of broilers at an early age. A total of 240 day-old male Arbor Acres (AA) broilers were randomly assigned to 4 groups, namely the control (CON) group (fed basal diets with 0 cfu/kg BS fmbj), the BS-1 group (fed basal diet with 0.2 × 1011 cfu/kg BS fmbj), BS-2 group (fed basal diet with 0.3 × 1011 cfu/kg BS fmbj), and BS-3 group (fed basal diet with 0.4 × 1011 cfu/kg BS fmbj). No differences were found in the average daily gain (ADG), average daily feed intake (ADFI), and feed conversion ratio (FCR) among the different treatments at 21 d. The BS groups showed lower (P < 0.05) levels of serum alanine aminotransferase (ALT) and aspartate aminotransferase (AST) than that in the CON group at 21 d of growth. The dietary BS fmbj increased (P < 0.05) the antioxidant activity in the serum, liver, and hepatic mitochondria, but decreased (P < 0.05) the serum and hepatic malondialdehyde (MDA) levels compared to those in the CON group at 21 d of feeding. The BS groups showed lower (P < 0.05) level of hepatic mitochondrial reactive oxygen species (ROS), protein carbonyls (PC), and 8-hydroxy-2-deoxyguanosine (8-OHdG); however, in this group, higher (P < 0.05) levels of mitochondrial membrane potential (MMP), adenosine triphosphate (ATP), and mitochondria DNA (mtDNA) were determined compared to those in the birds of the CON group at 21 d. The BS group showed increased (P < 0.05) levels of antioxidant related-gene expression in the liver and hepatic mitochondria compared to that in the CON group. In conclusion, BS fmbj (0.3 g/kg in broiler diets) has the potential to improve the antioxidant status of the body as well as the hepatic mitochondrial function and thus, appears to be an important additive for both the consumer and the industry.