Characteristics of the laminar now induced by reciprocating a disk impeller in a cylindrical vessel have been investigated by combining LDA velocity measurements with CFD calculations. From the variations of velocity fluctuation with Reynolds number, it is found that transition from laminar to turbulent now takes place at Re = 2 x 10(2). The now at 20 < Re < 2 x 10(2) is two-dimensional without producing tangential velocity component, and a vortex is generated behind the disk impeller. With increasing Reynolds number the center of the vortex departs from the disk edge and the vortex is elongated and strengthened in the rear region of the disk impeller. CFD calculations have been carried out for the case where the existence of the shaft is ignored. The velocity variations near the reciprocating disk are well represented by CFD calculations, which are symmetric between the upward and downward stroke except a small disk or small contraction of the flow through the gap. Influences of the narrow gap between the disk edge and the vessel wall and those of the shaft on the force acting on the disk are investigated by comparing the calculations with the measured data.