A twofluid model has been used to study the influence of the existence of the directive ions and directive electrons on the Bohm criterion of sheath. The main collisions included in this study consist of the charge exchange collision, ionization collision and total energy loss collision. The operated gas is argon with a gas pressure 101325 Pa. Numerical calculation results are obtained, which show that the minimal speed at which drift ions enter into the sheath for directive ions is decreased. This is also valid for the charge exchange collision. The directive electrons make this speed increase, but the ionization collision related to directive electrons makes the speed decrease obviously.