Abstract: It has been received much attention that the dispersionphenomena can be produced by unsteady flows in a channel during the past fewdecades because of their usages in chemical, biochemical and medicalengineering. As the study of fluid mechanics has evolved from steady stateinvestigations towards the reality of unsteady phenomena, it has becomeincreasingly clear that instantaneous whole field non-intrusive flowmeasurement techniques are required. In the past few years particle imagevelocimetry (PIV) has developed rapidly as a means for obtaining wholefields of fluid velocities. It has capitalized on recent computer hardwareand software developments, to the extent that it now is practicable fordetermining and mapping fields of unsteady movement, especially at thenarrow flow channels.This study makes the use of PIV to capture the vortex wave motion and theirstructures during oscillatory flows through a two-dimensional asymmetricchannel. The vortex wave is a relatively recently discovered fluid structurewhere a standing wave of vortices can be generated either by an unsteadymotion of a channel wall or by unsteady flow through an asymmetric channelexpansion. The vortex wave, which evolved at low speed, can enhance the masstransfer of boundary layer in channels and consequently maintain very lowwall shear rates for some shear sensitive microorganism or mammalian cells.In this paper, a set of experimental equipment was established for vortexwave flow. Unsteady incompressible flow, flowing at low-Reynolds-number inperiodic oscillatory pattern, in a two-dimensional channel with backwardsidestep structure was investigated by using PIV. The generation,development and disappear of the vortex wave about oscillatory flow in anasymmetric channel has been analyzed quantificationally. A series ofvelocity vector figures at different phases have been obtained. The velocityprofile and the vorticity of vortex wave have been tested and calculated inthis paper in order to analyze the mechanisms of strengthening masstransfer. The results show that the vortex wave flow-field is periodical andthe waves will be brought as the fluid periodically flows. It indicates thatthe main feature of vortex wave is that it is a two-dimensional standingwave formed during the deceleration period and the core flow follows acurving path with a sequence of vortices forming alternately on each wallbetween the core flow and the walls of the channel. In addition, therelationship between Reynolds number and vortex wave has been investigated.The experimental results corresponded satisfactorily with the analysingresults of the vorticity equation. It has showed that the vortex strengthenhances but the size of the vortices and the wave is almost changeless asReynolds number increases.