The effects of free-stream turbulence on the drag coefficient of a circular cylinder were investigated experimentally in an air duct. The experiments were performed over a range of Reynolds number, based on mean velocity and cylinder diameter, from 1,350 < Re < 8,000. A constant temperature hot-film anemometer was used to determine the turbulence intensity which was between 1.2% < u' < 21%. The range of the dimensionless turbulence scale was from 0.5 < ∂/D < 3.3, in which ∂ = the turbulence scale defined through the integral of auto-correlation function and D = the diameter of the cylinder. Two smooth circular cylinders were used. They were 1/4 in. and 1/2 in. 6.35 mm and 12.7 mm shell fraction. Quantitative results were obtained. Equations describing the relationship between the drag coefficient and turbulence characteristics were also obtained.