An experimental study was carried out to examine the behavior of the unsteady boundary layer. An NACA 0012 airfoil with aspect ratio of 2.7 was set vertically in a test section, which is sinusoidally pitched about the quarter chord. The oscillating amplitude is from -6$^{circ}$ to +6$^{circ}$ and the mean angle of attack is 0$^{circ}$. Surface mounted probes (Glue-on probes) were employed to measure the surface flow of the boundary layer. Measurements were made at free-stream velocities of 1.98, 2.83, and 4.03m/s, and the corresponding Reynolds numbers based on the chord length were 2.3$ imes$104, 3.3$ imes$104 and 4.8$ imes$104, respectively. The reduced frequency is fixed as 0.1 in all cases. The results show that the surface position of minimum shear stress and of boundary layer break-down can be discerned in the Reynolds number between 2.3$ imes$104 and 3.3$ imes$104.