Measurements of power requirement and mixing time have been conducted in a cylindrical vessel installed with a disk impeller reciprocating up and down. Mixing time was measured by using the decolorization reaction of iodine with sodium thiosulphate. As the Reynolds number specially defined for this reciprocation system increases, the dimensionless mixing time, which is the product of mixing time by reciprocation frequency, decreases and approaches a constant value. Mixing was completed within several reciprocations when the flow is turbulent with vortex generation behind the moving disk. The power number, P-o, was correlated with Reynolds number, Re, without showing any dependency on the ratio of impeller diameter to vessel diameter: P-o = 44 Re-1 for Re < 20 and P-o = 1 for Re > 200. The rate of recipro-mixing is compared with that of ordinary rotating mixing under the same power input per volume, P-v. In order to clarify the mechanism of rapid recipro-mixing, stretching and folding behaviours of a fluid interface between two miscible liquids were investigated by visualization with LIF. The fluid interface stretches exponentially with time in laminar flow with vortex generation, which indicates that reciprocation of a disk produces a chaotic mixing filed. Chaotic mixing together with strong turbulence caused by reciprocation yields rapid mixing that is not achievable by the ordinary rotating mixing.