This chapter investigates the stability control, fault detection, and estimation problem for uncertain high-order nonlinear systems with unknown system power. Firstly, a new adaptive controller is designed such that the system is stable and the state converges to the origin. Secondly, fault detection observer is constructed to detect the fault in time. Then, fault estimation observer is designed such that the fault can be estimated accurately in time. Comparing the works in literature, the control designing in this chapter overcomes some errors and relaxes some assumptions. Furthermore, novel observers do not require the condition that the input power is known, which is necessary in the observer design of the common system. Finally, an example is provided to validate the theoretical results.