An interesting flow phenomenon in a cross-flow impeller --- an asymmetrical throughflow in a geometrically symmetrical impeller --- has been investigated experimentally. The causes of the throughflow are explained in relation to the flow mechanisms. Flow was visualized by moire method of topography and solid particles. The transient process of the flow from the starting of an impeller until formation of a large eccentric vortex was observed in details. If Reynolds number is larger than 250, the throughflow occurs in all the types of the impellers experimented. In the steady rotation of the impeller the eccentric vortex rotates slowly around the impeller-axis and flow pattern changes sequentially. The change of flow patterns is essential to the flow of the cross-flow impellers.