This study was conducted to investigate the effects of dietary supplementation with calcium montmorillonite (Ca-MMT) on layers’ oxidation status, immune response, serum biochemistry, performance, and egg quality. A total of four hundred-eighty 75-wk-old laying hens were randomly assigned to 5 treatments with 8 replicates per treatment and 12 hens in each replicate. The hens were fed the basal diet supplemented with 0, 0.3, 0.6, 0.9, or 1.2 g Ca-MMT/kg for 70 d. The Ca-MMT supplementation linearly or quadratically (P < 0.05) increased yolk index, serum total superoxide dismutase activity, and interleukin-2 and immunoglobulin G concentrations at d 35, and shell thickness, and serum and liver glutathione peroxidase activity at d 70. With increasing supplementation of Ca-MMT in the basal diet, serum activity of alkaline phosphatase decreased both linearly and quadratically (P < 0.05) at d 35, and alanine aminotransfer as activity and tumor necrosis factor-alpha concentration decreased linearly or quadratically (P < 0.05) at d 70. The evidence indicates that increasing Ca-MMT concentration improved yolk index and shell thickness, and partially enhanced hens’ antioxidant capability and immune function.