This study deals with fluid mixing process in the circular cross-section of a cylindrical vessel using a rotationally reciprocating (back and forth rotation) plate impeller from the view point of experimental visualization and numerical simulation. We have investigated the fluid deformation process with the visualization of streak lines. It has been found that a pair of vortices generated at the impeller tip plays a crucial role in stretching and folding of streak lines. Additionally, for effective overall mixing, we found that fluid needs to be dragged inward to the region near the mixing shaft, stretched and folded along the impeller plate, and then extensively deformed from the impeller tip. However, since the path of fluid inward motion is rarely disturbed by locally strong vortices generated at the conditions of small amplitude and short period, a poor mixing region expanded from the mixing shaft to the vessel wall, which made two separated mixing regions in a vessel. With increasing amplitude, the poor mixing region shrunk near the mixing shaft. In the case of the largest amplitude and longest period, fluid was effectively deformed everywhere in the vessel, which leads to a highly intensified mixing process.