The effect of gradually increasing supplemental levels of blueberry extract on growth performance, carcass characteristics, and fatty acid composition of breast and thigh muscles of broiler chickens was investigated. One hundred ninety-two 7-day-old chickens were randomly distributed into four groups having four replicates with 12 birds in each replicate. Basal diets were prepared for starter (days 8 to 21) and finisher (days 22 to 42). Basal diets were offered to the control group only, whereas other treatments received basal diets fortified with 0.5, 1, and 2% blueberry extract (BB0.5, BB1, and BB2 groups, respectively). The duration of experiment was 35 days (days 8 to 42). During finisher and overall growth phases, broilers in the BB2 group had greater body weight gain than those in the BB0.5 and control groups, whereas the BB1 group had higher body weight gain than the control group (P < 0.001). Body weight gain remained unaffected during the starter phase. Feed intake was greater in the BB2 group than in the control group at days 8 to 21, 22 to 42, and 8 to 42 (P = 0.002, P = 0.035, and P = 0.001, respectively). The control group had poor FCR than the BB2 group in the starter phase (P = 0.034). At days 22 to 42, feeding blueberry extract (BB0.5, BB1, and BB2) improved the FCR of broilers compared with the control group, whereas the BB2 group had better FCR than the BB0.5 group (P < 0.001). At 8 to 42 days, broilers in the control group had poor FCR compared with the BB1 and BB2 groups, whereas the BB0.5 group had poor FCR than the BB2 group (P < 0.001). Slaughter weight was lower in the control group than in the blueberry extract groups, whereas the BB2 group had greater slaughter weight than the BB0.5 group (P < 0.001). Dressing percentage of broilers in the control and BB0.5 groups was lower than that in other groups (P < 0.001). Gizzard yield was higher in the BB0.5 and BB2 groups than in the control group (P = 0.021). In addition, feeding 2% blueberry extract increased the concentration of different fatty acids in breast and thigh meat of broiler chickens. Findings suggest that feeding 2% blueberry extract may improve growth performance, carcass characteristics, and fatty acid composition of breast and thigh muscles of broilers.