This experiment was conducted to investigate the effects of dietary vitamin D level on immune and antioxidant functions of egg-type ducklings aged 1 to 28 days, and to estimate the optimal dietary vitamin D level. A total of 180 one-day-old healthy Jinding egg-type ducklings with the similar body weight were randomly assigned to 5 groups with 6 replicates per group and 6 ducklings per replicate. The ducklings in the group Ⅰ (control group) were fed a basal diet (corn-soybean meal-based diet), and those in groups Ⅱ to Ⅴ were fed the basal diet supplemented with 110, 220, 550 and 1 000 IU/kg vitamin D, respectively. The experiment lasted for 4 weeks. The results showed as follows: supplementation of 550 IU/kg vitamin D significantly increased the thymus and spleen indices (P<0.05), and improved the activities of superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px) in liver (P<0.05), while reduced the contents of interleukin-2 (IL-2) in serum and malondialdehyde (MDA) in serum and liver (P<0.05). Supplementation of 220 and 550 IU/kg vitamin D both significantly increased the contents of total protein (TP) and albumin (ALB) in serum (P<0.05), and improved the activities of SOD and GSH-Px in serum (P<0.05) and total anti-oxidant capability (T-AOC) in serum and liver (P<0.05). These results indicate that corn-soybean meal-based diet supplemented with moderate level of vitamin D can increase the egg-type ducklings immune and antioxidant functions. Based on the various indicators, the optimal vitamin D supplemental levels estimated by quadratic regression model analysis are 512.8 to 550.0 IU/kg.