Experiments on mixing performance have been carried out by reciprocating a disk in a cylindrical vessel, The force acting on the disk was measured by a force transducer installed between the impeller shafts, and the disk position was detected by a photo interrupter. With increasing reciprocating Reynolds number, the dimensionless maximum force acting on the disk decreases in inverse proportion to the Reynolds number in laminar creeping flow and becomes almost constant in turbulent flow, The Lissajous figure of force and disk position during reciprocation is deformed from an ellipsoid to an inclined parallelogram as the flow changes from laminar to turbulent. The average power is obtained by integrating the product of force and disk velocity during one cycle. The power number can be correlated with the reciprocating Reynolds number in a similar way to correlating the maximum force, The mixing process was visualized by using the decolorizing reaction, based on which the flow behavior could be divided into three patterns; laminar creeping flow without vortex generation, laminar flow with generation of a vortex which dissipates within a half cycle of reciprocation, and turbulent flow with vortexes. The mixing process is extremely slow in laminar creeping flow. It is promoted by stretching and folding of fluid lumps in the flow with vortex generation. The dimensionless mixing time was investigated in terms of the power input per unit volume of liquid, In turbulent flow, the mixing time is considerably smaller than that required for ordinary rotating mixing.