This study was conducted to investigate the effects of naturally oxidized corn oil on the inflammatory reaction and intestinal health of broilers. Total 450, one-day-old Arbor Acres male broilers were randomly divided into 5 treatments with 6 replicate cages (15 birds in each replicate cage). The dietary treatment array consisted of the varying ratio of nonoxidized corn oil to naturally oxidized corn oil from 0:100, 25:75, 50:50, 75:25, and 100:0, respectively. The experimental period was 42 d. Serum, jejunum, and contents of cecum samples were taken at the age of 42 d of broilers. The results showed no significant difference in the body weight gain (
BWG
) with a different proportion of oxidized corn oil compared with the 0% oxidized oil group on d 42. The feed intake (
FI
), the concentration of immunoglobulin G (
IgG
), interferon-γ (
IFN-γ
), and interleukin-10 (
IL10
) in serum showed a significant quadratic response with the increase of oxidized oil concentration on d 42. The serum's concentration of IgG, IFN-γ, and IL-10 reached the highest value at 75% oxidized corn oil. In addition, the mRNA expression levels of interleukin-1β (
IL-1β
), IFN-γ, nuclear factor kappa B (
NF-κB
), tumor necrosis factor α (
TNF-α
), and myeloid differentiation factor-88 (
MyD88
)
in the jejunum were significantly affected by different proportions of oxidized corn oil, and the gene expression levels were highest at 75% oxidized corn oil on d 42. The mRNA expression of Bcl2-associated X (
Bax
) in the jejunum showed a significantly quadratic curve with the increase of oxidized oil concentration, and its gene expression was the highest after adding 50% oxidized corn oil according to the regression equation on d 42. The villus height/crypt depth and goblet cells of jejunum decreased linearly with the increasing proportion of oxidized corn oil and reached the lowest point after adding 100% oxidized corn oil on d 42. The β diversity showed the remarkable differentiation of microbial communities among 5 groups, and the microbial community of the 0% oxidized oil group was significantly separated from that of 75 and 100% oxidized oil groups in the cecum. Taken together, these results showed that a low dose of naturally oxidized corn oil is not harmful to the growth of broilers, while a high dose of oxidized corn oil will trigger the inflammatory response and adversely affect the gut health of broilers.