Background: Vitamin C is an essential nutrient for teleost fish. It plays vital role in immune system function. OBJECTIVES: A feeding trial was conducted to determine the effects of different levels of dietary vitamin C supplement on growth performance, hematological indicators and response to heat stress in rainbow trout (Oncorhynchus mykiss) fingerlings kept at different densities. METHODS: The fingerlings (initial weight of 2.1 ± 0.1) were reared in triplicate groups in fiberglass tanks at two different densities (400 and 600 /m2). The fish were fed with four experimental diets containing different levels of vitamin C (0, 100, 200 and 1000 mg vitamin C /kg diet) for 9 weeks. To evaluate the effect of heat stress on survival, fish were exposed to different temperatures (24, 26 and 28 °C) at the end of feeding experiment. RESULTS: Growth performance in fish which fed diets containing different levels of vitamin C (100, 200 and 1000 mg /kg diet) was significantly (p<0.05) higher than those fed diet without vitamin C supplement. In terms of hematological indicators, it was shown that different levels of vitamin C significantly (p<0.05) effected the hematocrit, hemoglobin and red blood cell (RBC) count. However, it did not make any effect on the MCH and MCHC values. On the other hand, stocking density only had significant effect on RBC count. CONCLUSIONS: Survival of fish which fed diets containing different levels of vitamin C was significantly (p<0.05) higher than those fed diet without vitamin C in response to heat stress.