An experimental study was carried out to investigate the mixing process of a two-layer stratified fluid in a laterally heated enclosure. Due to the lateral heating of the enclosure, a circulating flow is induced in each layer such that the interface separating the layers is simultaneously exposed to destabilizing sheer and double diffusive convection. The main goal of this work is to investigate the role of the interfacial instabilities in the mixing process of the two-layer system. The experiments were carried out in a box with inner dimensions of 10 x 10 x 10 cm. Two sidewalls of the box were made of stainless steel and served as heat exchangers, and the two other sidewalls were made of optical glass to facilitate flow visualization. The criterion for the onset of interfacial instabilities and the mixing time of the system were studied experimentally. The results show that when the flow adjacent to the interface is unstable, it is characterized by intense vortices and the mixing time is relatively short. On the other hand, when the interfacial flow is stable, no vortices exist at the interface and the mixing time is much longer.