This paper makes a study of the problem of guaranteed cost control in chaotic systems, and obtains a necessary and sufficient condition existing in guaranteed cost controllers. The result shows that this condition is equivalent to the feasibility problem of a certain linear matrix-inequality system, and provides a guaranteed cost controller. The design problem of optimal guaranteed cost controller in chaotic systems is formulated as a convex optimization problem on the basis of this, which can be solved by convex optimization techniques. Finally Chen system is taken as an example to demonstrate the effectiveness of this method.