This paper presents a terrain-adaptive wheel-track hybrid robot that uses a pair of combined wheel and track systems. The hybrid mobile platform can change the shape of track to adapt to various terrains so that it is able to move fast on flat terrain and to show good performance in overcoming stairs or obstacles. The proposed platform consists of an ordinary wheel structure for fast navigation on flat floors and a transformable tracked structure for climbing stairs effectively. In detail, three track arms installed on each side of the platform are used for the navigation mode transition between flatland navigation and stair climbing. The mode transition is determined and implemented by the adaptive driving mode control of the mobile robot. This wheel-track hybrid mobile platform is evaluated through experiments that assess its navigation performance in real and test-bed environments. This hybrid mobile robot is embodied to perform given tasks in a hazardous environment for surveillance, reconnaissance, and search and rescue applications.