Thick airfoils are commonly employed at the root of wind turbine blades to improve the structural strength; nevertheless, they are vulnerable to flow separation at low Reynolds numbers due to low momentum at boundary layer. A Humpback whale is a giant animal that has excellent maneuverability in water. It has tubercles at its flipper (wing) that are nearly sinusoidal shaped. In the recent two decades, flow control through the tubercled leading edge (TLE),mimicking the humpback’s flipper, has been intensively investigated; however the size of the tubercle depends on specific airfoil used. In this thesis, the effect of tubercle size, particularly amplitude, on aerodynamic performances of S821 airfoil was investigated considering a 5kW horizontal axis small-scale wind turbine (SWT). For this, the baseline (unmodified) airfoil with three different ratios of amplitude, 2.5%, 5%, and 12% of the mean chord length, at low Reynolds number (2.18x10 5 ) was studied by an experimental wind tunnel test and computational fluid dynamics (CFD) using a FLUENT solver at a range of angles 0°-28°. The experimental results showed that small reduction of lift coefficient and an increase in drag of TLE-modified airfoils at angles lower than the pre-stall angle of baseline (16°) which was a drag penalty. However, the performances of all the three TLE airfoils were better than the unmodified airfoil above 16°. The smallest amplitude airfoil showed the least performance from TLE airfoils, but much higher than the baseline airfoil. The medium amplitude TLE airfoil (5%C) showed an 11.93% greater lift than baseline at an angle of attack (AOA) of 28° and delayed stall angle by 25%. The coefficient of pressure (𝐶 � ) distributions over the airfoils showed that the trough cross-section experienced the largest suction negative 𝐶 � values compared to the peak cross-sections for TLE airfoils. The CFD results indicated a good correspondence to the experimental tests such that the airfoil with medium amplitude experienced the best post-stall behavior such that it delayed stall angle by 17.5% with a post-stall lift increment of 15.9% at 28°. Keywords: Tubercle, wind turbine, airfoil, flow separation, whale flipper, stall, amplitude