A data acquisition technique is adapted to obtain three dimensional velocity distributions of the flow in a 180°-bend square duct by combination of a hydrogen bubble method and an image processing technique. The results show the existence of the flow toward inner wall immediate upstream of the inlet to the curved channel and of the additional secondary flow near the outer wall of the curved section. These characteristic phenomena lead to the three dimensional distortion of mean flow development. Moreover, the results obtained by this method were compared well with the calculated results of the elliptic N-S equation for this flow field. The present data acquisition system is suitable to this kind of flow.