The characteristics of a pressure-driven water flow including flow micro-structures and pressure drops are investigated in serpentine micro-channels with miter bends. The micro-channels have rectangular cross-sections with hydraulic diameters of 0.209 mm, 0.412 mm and 0.622 mm, respectively. To segregate the bends and entrance effects individually from the total pressure drop, for each size three types of micro-channels: straight short, straight long, and long serpentine, were fabricated to get the reliable pressure data without entrance effect. A micron-resolution particle image velocimetry system (micro-PIV) was develop and used to obtain the detailed velocity vector field. The experimental results show that the vortices around the outer and inner walls of the bend do not form when Re < 100. Those vortices appear and continue to develop with increasing Re number when Re is larger than a value around 100, and the shape and size of the vortices almost remain constant when the Re is larger than a value around 1000. The experimentally observed additional pressure drop due to the bend is commensurate with the strength of the calculated vortices. The bend loss coefficient K-b was observed to be related only with the Re number when Re < 100, but with the Re number and channel size when Re 100. It almost keeps constant and changes in the range of +/- 10% when the Re is larger than a threshold value somewhere in 1000-1500. (c) 2007 Published by Elsevier Ltd.