Cross shaped baffles are set in the horizontal drum to augment mixing of the size-type binary particles, and the discrete element method was employed to study the mixing of the size-type binary granular systems in the rotating horizontal drums with different sized cross shaped baffles. The influences of the baffle and the size of the baffle on the mixing were discussed. The mechanisms of the augmenting of mixing of the baffles were discussed in this paper. The simulation results show that the convection, the diffusion and the segregation controlled the mixing and demixing of the binary granular systems in the rotating horizontal drum without baffle. When the baffle was set in the drum, only the convection and the diffusion operated, and the segregation of the binary particles were suppressed, because the cross shaped baffles can break the free-surface flow in the drum and the segregation of the binary particles in the free-surface flow can not occur, and the mixing can be enhanced. An optimum size of the baffle exists for the augmenting mixing of the size-type binary granular systems in the drum with baffles. The oversized and undersized baffles are unfavorable for the mixing of binary particles in the drum.