The main objectives of the present study are to visualize a bubbly turbulent boundary layer and to investigate the role of the bubbles in frictional drag reduction. The turbulent boundary layer is formed under the surface of a 2-D flat plate. A microbubble generator is used to produce bubbles with diameters in the range 5-100 mu m and a mean bubble diameter of 30 similar to 50 mu m. The behaviors of the microbubbles are visualized quantitatively by using the conventional PIV technique with a field-of-view of 200 mm(2). The velocity fields of the bubbles show that they reduce the Reynolds stress in the boundary layer. To understand the relationships between the distribution of the void fraction due to the microbubbles and the frictional drag reduction, the shadowgraphy technique was adopted within a tiny field-of-view of 5.6 mm(2). The bubble images are analyzed by performing shadow processing to obtain information about their shapes and speeds. A local 2D void fraction of approximately 3.5% in the buffer layer is found to be highly effective in reducing skin friction by decreasing the Reynolds stress. The 3D void fraction of 1.3-3.0% is also found to reduce skin friction in the present study. The vertical fluctuating motion of microbubbles as well as a high concentration of them in buffer layer is effective in reducing the skin friction in high Reynolds number regime of 10(6). (C) 2015 Elsevier Ltd. All rights reserved.