Abstract: Presents the design of variable structure control law on sliding mode for Acrobots as a class of underactuated mechanical system, with a modified exponential approximation law employed in the design. The simulation results showed that an Acrobot system remains stable if the variable structure controller designed in terms of linear model is employed in nonlinear system. Comparing with the control by pole placement method, a bigger stability range (or basin of attraction) and stronger robustness can be provided for an Acrobot system by the balance controller which is designed by variable structure control based on sliding model. The modified exponential approximation law can not only reduce effectively the tremble caused by variable structure control but make the Acrobot system come to a stable equilibrium state quickly.