To clarify the inside flow structure in a sphere-packed pipe, PIV visualization is conducted first by utilizing a matched refractive-index method with Nal solution as the working fluid. As the basic flow structure in the pipe, the Following three flows are confirmed: the bypass flow with high flow velocity due to wall effect, the complicated wake formed behind the sphere and the spouting flow from the central part of pipe. Furthermore, through heat transfer experiment, wall-temperature distribution is measured with thermocouples and an infrared thermography, which clarifies the relation between the flow structure and local heat-transfer performance. The area with high wall-temperature is formed in the stagnation area located around at a contact point between the sphere and the heating wall. However, the heat transfer performance is quite high in the area with a large gap between the upstream and downstream spheres by means of the influence of a separation vortex and an impinging flow, which are both a part of complicated wake. In addition, the impinging effect of the high-velocity bypass flow on the spheres significantly affect on the heat transport in the stagnation area.