Interface roughness effects on direct tunneling current in ultrathin MOSFETs are investigated by numerical analysis. The interface roughness is described in terms of Gauss distribution. It is shown that the effects of rough surface on tunneling current cannot be neglected while tunneling occurs in the regime of direct tunneling. The effects increase exponentially with oxide thickness or applied voltage across oxide decreasing. This means that the effects become more and more important while the direct tunneling current takes up the main contribution to the gate leakage current.