In this paper, a robust sliding mode control for a flexible transmission system is presented. This system has two very oscillatory vibration modes subject to large load variation which makes the control of it be difficult. A set of design specification in time domain is to be met by a single controller on each of the three main plant models. Furthermore the tracking problem in the presence of disturbance and load variation is investigated. Results show good performance of the proposed controller. In this approach, in order to estimate the hidden states of the flexible transmission system, an observer based on sliding mode control is used.