This research aims to investigate the antioxidative potential of dietary chitosan (CH) and nano-chitosan (NCH) supplementation on the gene expression of pivotal antioxidant defense system components, namely SOD (superoxide dismutase) and CAT (catalase) genes. These genes are instrumental in safeguarding cells against the deleterious effects of reactive oxygen species. The study extends its scope to evaluating the hepatic and intestinal histology in two distinct lines of Japanese quail. Carried out as a fully randomized design experiment with 10 treatment groups, the research encompassed 840 one-day-old Japanese quails from two distinct lines: Sq, selected for body weight at 30 days after 42 generations of selection, and Jq, a control line without selection. Each line was further divided into five treatment groups, with three replicates per group and 28 unsexed chicks per replicate. The treatment groups included a baseline diet according to NRC (1994) guidelines, two groups supplemented with chitosan (CH) at 50 and 70 mg/kg diet, and two groups supplemented with nano-chitosan (NCH) at 30 and 50 mg/kg diet for both Sq and Jq lines. The study outcomes shed light on the varying concentrations of chitosan (30, 50, and 70 mg/kg) and their impacts on live body weight, CAT and SOD gene expression levels, as well as histopathological changes in the liver and intestine. The findings revealed discernible effects across all tested concentrations, elucidating the multifaceted influence of chitosan and nano-chitosan supplementation on the examined parameters.