This paper addresses the problem of fault detection and isolation (FDI) for a mini-quadrotor. First a model for a four-rotor rotorcraft is presented whose model is obtained via a Lagrange approach. In order to stabilize the quadrotor at low cruise speed, a control strategy based on PD (Proportional-Derivative) controllers is presented. Using a Thau’s observer, a diagnostic system has been developed for the nonlinear model of the quadrotor. Different simulation trials have been performed and the analysis of the results proves that the developed diagnostic system represents an effective solution to FDI problems in mini-flying machines.