Cinnamon essential oil (CEO) and its combination with bamboo leaf flavonoid (BLF) have been shown to exhibit an additive antibacterial effect in vitro, but their functions in broilers were not clear.  An experiment was conducted to investigate the effects of dietary graded levels of CEO and its combination with BLF on the growth performance, immune responses, antioxidative ability, and intestinal morphology and microbiota of broilers fed a corn-soybean meal basal diet.  A total of 576 1-d-old Arbor Acres commercial male broilers were randomly allotted to 9 treatments with 8 replicates of 8 birds each in a completely randomized design.  Birds were fed on a basal corn-soybean meal diet (control, without plant extracts and antibiotics), or the basal diet supplemented with 50 mg of aureomycin kg^–1, 50, 100, 200, 400, or 800 mg  of CEO kg^–1, a combination of 100 mg of CEO and 16.7 mg of BLF kg^–1, or a combination of 200 mg of CEO kg^–1 and 33.3 mg of BLF kg^–1 for 42 d.  Dietary treatment affected (P<0.05) the serum immune globulin M (IgM) contents on d 42, liver malondialdehyde (MDA) contents on d 21, duodenal crypt depth on d 42, relative abundance of Lactobacillus in the cecal contents on d 21, and relative abundances of Escherichia coli and Bifidobacterium in the cecal contents on d 42, but had no effect (P>0.16) on all other measured indices.  The addition of 400 mg of CEO kg^–1 or a combination of 200 mg CEO kg^–1 and 33.3 mg BLF kg^–1 increased (P<0.02) serum IgM contents on d 42.  Dietary supplementation with 100 or 200 mg CEO kg^–1, or 50 mg aureomycin kg^–1 decreased (P<0.003) liver MDA contents on d 21.  In addition, the supplement of 100 mg CEO kg^–1 increased (P<0.002) the Lactobacillus relative abundance in caecum on d 21 and Bifidobacterium relative abundance in caecum on d 42, and decreased (P<0.0001) E. coli relative abundance in caecum on d 42.  The results indicated that dietary supplementation with CEO, an alternative to aureomycin, improved the immune status, antioxidantative ability and cecal microbiota of broilers, and dietary supplementation with the combinations of CEO and BLF did not exhibit further effects.  Dietary supplementation with 100 mg CEO kg^–1 is beneficial for broilers fed a corn-soybean meal basal diet.