This research was conducted to evaluate the efficacy of vitamin C-polyethylene glycol complex (VCPEG) in broiler chickens during heat stress. For a 6-week feeding trial, a total of 330 broiler chickens were allotted to five dietary treatments. The dietary vitamin C (VC) levels (treatments) were, mg/kg: 1. 0 VC (negative control), 2. 200 VC as ascorbate (positive control), 3. 10 VC-PEG, 4. 20 VC-PEG, and 5. 50 VC-PEG. At the end of the experiments nine chickens per treatment were sacrificed for the analyses of VC contents in blood and liver, and meat quality. During the starter period, chickens on negative control group (no VC added) grew slower (P<0.05) than others. During the overall period, chickens fed diets containing 20, 50 mg/kg VC-PEG, and 200 mg/kg ascorbate showed improved weight gain (P<0.05) than those fed 0 or 10 mg/kg VC-PEG. VC concentrations in plasma and liver were higher in chickens fed diets with vitamin C. Bone strength was also higher (linear, P=0.001) as dietary VC level was increased. There was no effect on lymphocyte sub-population. Antibody response to Newcastle disease and infectious bronchitis vaccination was higher in 20 mg/kg VC-PEG fed group as compared with other VC-PEG groups. In conclusion, VC supplementation was beneficial to broilers during heat stress.