Experimental studies were carried out on the instability of laminar-jet diffusion flames impinging on a cylinder. This instability implies the oscillation of flames near the stagnation point, where behaviors of flames are divided into three according to the distance from nozzle exit to the cylinder, impingement length(L). As impingement length increases, flames under the cylinder become stable first, unstable next and stable again. To investigate the characteristics of the instability, the frequency of the oscillation was measured and planar visualization was conducted. It is shown that the oscillation frequency depends on impingement length, cylinder diameter, nozzle width and nozzle exit velocity. Self-sustained oscillations of impinging free shear layers at non-reactive flow fields was investigated and compared with oscillations of flames. Also, it is visualized that the cylinder inserted into flames has an effect on the amount of smoke production according to impingement length.