Abstract: For lag synchronization of uncertain fractional-order chaotic (hyperchaotic) systems,a new sliding mode controller that combines active control with sliding-mode control is proposed. This also includes a fractional-order term. On the basis of the Lyapunov theory and stability theorem of fractional-order systems,the stability of the controlled system is analyzed. Two experiments namely lag synchronization between fractional-order chaotic Lü system and fractional-order chaotic Liu system and lag synchronization between fractional-order hyperchaotic Lorenz system and fractional-order hyperchaotic Chen system,are performed successfully. The numerical simulations illustrate effectiveness and robustness of the proposed method.