This paper studies chaos synchronization of the new hyperchaotic Chen system via nonlinear control. A new method for improving the Lyapunov function of error dynamics of synchronization designed by Jiang and Huang et al is presented. The method removes the limitation occasioned by the Lyapunov function having only one form. Based on Lyapunov theory, a nonlinear controller is designed to synchronize two identical systems and two different chaotic systems. Chaos synchronization of two identical hyperchaotic Chen systems and two different chaotic systems (the hyperchaotic Chen system and the hyperchaotic R?ssler system) have been proved theoretically. Numerical simulations show the effectiveness of the proposed method.