In this paper, nonlinear observer design for a chaotic system has been investigated. Indeed, a high gain observer and a sliding mode observer are proposed to the Colpitts circuit. The synthesis of these observers took place under certain sufficient conditions arising directly necessary and sufficient conditions established for nonlinear systems. Comparing with the results obtained by these observers, the high gain observer showed a good ability to reconstruct the states of the chaotic system and providing a small mean squared error compared to that of the sliding mode observer.