In this paper, the segregation patterns of size-type binary granular systems with different damp coefficients in a rotating horizontal drum are simulated by the discrete element model. The process and the mechanism of segregation are studied, and the effects of damping on segregation model of size-type binary granular system in a rotating horizontal drum are discussed. The simulation results show that the segregation process and the flow pattern of granulae in a flowing layer depend strongly on damping. According to the simulation results we predict that the radial segregation patterns that form in a size-type binary granula system in a thin rotating horizontal drum will be the sun pattern when the damp coefficients is in a middle value range, otherwise, the moon pattern will appear.