The fluid model and the self-consistent dust charging model are used to investigate the Bohm criterion for the plasma sheath including two species of charged dust particles. Numerical calculation results are obtained using quasi-Newton method. Effects of two species of dust particles on the dust charging and the ion Mach number are discussed. As the number density of the fewer dust particles is enhanced, different changes will take place on the surface potential of the two species of dust particles. Either more number density or smaller radius of the fewer dust particle may result in a larger ion Mach number. In addition, the ion Mach number will reduce when the velocity of any kind of dust particles at the sheath edge increases.