For chaos control problem, in accordance with the constraints of control system energy, a quadratic objective function is first determined, and then a simple method for obtaining the optimal control law is proposed. This method gives the optimal control law by solving linear quadratic optimal control problem and avoids the difficulty of solving Hamilton-Jacobi-Bellman equation. Using Lyapnuov method, we prove the stability of the closed-loop system. For the unified chaotic system and Liu chaotic system the simulation results show the effectiveness of the control method.