Cubesat-class nanosatellites are characterized by their inexpensive cost of manufacturing and launch; thus, have gained a lot of interest in research recently. An actuator model for the three Magnetorquers and three reaction wheels is derived, as well as the kinematical and dynamical model of the Cubesat, for implementing an Attitude Determination and Control System (ADCS). Two different control approaches are investigated. Firstly, a Quaternion Feedback (QF) algorithm is derived and applied to assess the response of the system. Furthermore, a Sliding Mode Disturbance Observer-Based Control (SMDO) is implemented to achieve robustness against the un-modeled dynamics represented in the high coupling between the reaction wheels and the satellite dynamics. The SMDO controller has reduced the oscillations in the states response as well as the stabilization time in comparison to the QF controller. Finally, the effectiveness of SMDO has shown a significant decrease in the control effort compared to conventional sliding mode control.