Particle image velocimetry (PIV) experiments have been carried out to study the correlation between the high-Reynolds number turbulent flow and wall heat transfer characteristics in a two-pass square channel with a smooth wall and a 90degrees rib-roughened wall. Detailed averaged velocity distributions and turbulent kinetic energy for both the main and the secondary flows are given for a representative Reynolds number (Re) of 30,000. The PIV measurement results were compared with the heat transfer experimental data of Ekkad and Han [International Journal of Heat Mass Transfer 40 (11) (1997) 2525-2537]. The result shows that the flow impingement is the primary factor for the two-pass square channel heat transfer enhancement rather than the flow turbulence level itself. The characteristics of the secondary flow, for example, vortex's shape, strength, rotating-direction and positions, are closely correlated with the wall heat transfer enhancements for both smooth and ribbed wall two-pass square channels. The rib-induced flow turbulence increases the heat transfer mainly because of the enhanced local flow impingement near the rib.