This paper proposes a robust bang-bang controller (RBC) based on the high-gain nonlinear observer for a nonlinear system with intermittent disturbances. The intermittent disturbances are able to model the system faults and sudden changes of operating conditions of practical nonlinear systems. A bang-bang constant funnel controller (BCFC) is employed

and the largest control capabilities of the control system is explored to stabilize the nonlinear system against the disturbances. The BCFC functions with the estimates of tracking error of control target and its derivatives estimated with a high-gain state observer

which eliminates the requirement on the derivative calculations of the system output tracking error. The error convergence of the observer is verified. The closed-loop stability of the RBC is proved in a bounded-input bounded-state sense. Simulation studies are undertaken with a single-machine infinite-bus (SMIB) power system to test the performance of the RBC.