The effect of plasma bulk flow on the ion tearing mode occurring in the Earth's plasma sheet is investigated. The flow is assumed to be confined to the region of open field lines and aligned with the magnetic field. An explicit dispersion relation is obtained for two kinds of Alfv¿n Mach number profiles idealizing the plasma mantle flow and the plasma sheet boundary flow. In both cases, the growth rate as well as the region of unstable wave numbers are increased as compared to the case without plasma bulk flow. The effect of plasma mantle flow on the growth rate increases as the inner boundary of the mantle approaches the plasma sheet. For the case of plasma sheet boundary flow, the growth rate increases rapidly as the layer where shear flows are concentrated moves toward the center of the sheet. Possible consequences of these results for the dynamic properties of the magnetotail are discussed.