Velocity measurements have been conducted in order to examine characteristics of turbulent flow induced by reciprocating a disk impeller in a cylindrical vessel. Variations of the average velocity fluctuation show that the transition regime from laminar to turbulent flow is located at 2 x 10(2) < Re < 4 x 10(2). In turbulent flow regime, a circulating flow with radial and axial velocity components is generated in the rear region of the disk. At a moderate Reynolds number a secondary circulating flow is induced behind the original circulating flow, but at a sufficiently high Reynolds number the region of the original circulating flow elongates without generating such a secondary circulating flow. Turbulence level is much higher in the circulating now region than in the front region of the reciprocating disk. The fluctuating velocity components in the radial, tangential and axial directions show similar periodic variations to each other irrespective of the magnitude of the Reynolds number, and the maximum fluctuating velocities attains about the same magnitude of the maximum disk velocity. The flow pattern and the magnitude of turbulence change violently during one cycle of reciprocation. In addition, at a sufficiently high Reynolds number, circumferential circulation of the fluid appears in the front and rear regions near the disk, whose direction is opposite to each other. The excellent mixing performance of recipro-mixing in turbulent flow regime is reasonably explained by these findings.