The performance of air-cooled generators can be improved only if they have efficient system designs for heat removal. An air-cooled generator is composed of a pair of coaxial cylinders, namely, a fixed outer cylinder (stator) and a rotating inner cylinder (rotor); the rotor has axial slits. In this study, we experimentally and numerically clarified the flow behavior and the heat transfer characteristics of rotating coaxial cylinders by simulating a salient-pole rotor in an air-cooled generator. The flow behavior in the slit between the salient poles was observed by using a high-speed video camera. We measured the temperature on the slit walls to investigate the heat transfer. The velocity fields and the heat transfer coefficient between the rotor and the stator were obtained via a numerical simulation. From the results, we experimentally and numerically observed the vortex structure in the slit. The local Nusselt numbers on the front-side wall of the slits near the impinging flow were higher than those on the back-side wall near the separated flow. The local Nusselt numbers on the front-side wall were high because the gap flow between the cylinders impinged on the front-side wall and promoted heat transfer. By contrast, the local Nusselt numbers on the back-side wall were low because a separated flow appeared near the back-side wall, where the hot fluid was retained, thereby causing the separated flow to disturb the heat transfer on the back-side wall.