A study of the dynamics of ionospheric plasma convection in the postmidnight sector during the substorm expansion phase (EP) is presented. We have found that after the substorm onset, a postmidnight anticlockwise convection vortex (PoACV) usually emerges at latitudes higher than the auroral brightening region, and an east-to-west flow reversal wrapping around the intensified auroras extends into the postmidnight sector. The substorm current system inferred from the relative positions of the PoACV and the auroral brightening region is in general northeast-southwest aligned, implying a mixture of a meridional current system (MCS) and a zonal system associated with the substorm current wedge (SCW). The time delay between the formation of the PoACV and the onset depends upon the relative azimuthal location of the PoACV and the auroral brightening region. The observed postmidnight convection pattern, namely, a PoACV at higher latitudes, and an east-to-west flow reversal region at lower latitudes, can be explained by a combination of the nightside reconnection and the "field line slippage" process associated with dipolarization. Finally, we suggest that strong negative IMF By is the most unfavorable condition for the evolution of the PoACV.