Passive mode locking in quantum cascade lasers (QCLs) is one of great challenges because of the ultrafast relaxation time of QCL compared to conventional semiconductor lasers. Thus, the use of conventional saturable absorber as Semiconductor saturable-absorber mirror (SESAM) is not the best solution for QCLs mode locking. The study proposed in this paper concerns the use the single-layer graphene as a saturable absorber. The relaxation time in graphene is very fast compared to the pulse width and relaxation time of QCL and its saturation intensity is low compared to other saturable absorbers. The use of graphene with high reflection mirror allows to increase the internal electric field which easily reaches the graphene saturation level.