standard sliding mode controllers are aclass of variable structure control systems which due to robustness have found their way to control societies .beside outstanding capacities of these controllers, some disadvantages arises that have been suppressed in new generations of these controllers.on the otherhand underactuated systems are systems with fewer inputs than their DOF's. for synthesis and control them, we need to transform them into normal forms.accordingto existing litrature, using partial feedback linearization and legandre transformation, two DOFs systems may transform into cascade form with two subsystes; internal dynamic and double integrator subsystems. in this thesis in order to regulate a class of two DOF underactuated systems, sliding mode controller is employed in a unusual manner, distinctively to change the structure of system. however to stabilize both subsystems successfully a variable based on the concept of differential flatness is introduced which can affect both DOFs. also the condition in which legandre transformation is a flat output is released and a feedback is developed which transforms the input. by this trick system is transformed into linear controlable canonical form. eventually sliding mode controller is applied. finally for this procedure an algorithm is generated and to examine the efficiency of algorithm, it is applied to RTAC benchmark and its robustness is inspected against parametric uncertainties and disturbance and finally is compared with a hybrid controller.