The cellular and dendritic formations are two kinds of typical morphology in the solidification, and there are many theoretical models and experimental researches on them. Most models and researches are based on purely diffusive transport mechanism. However, convection effects are of importance in the evolution of cellular and dendritic growth. Since the metal materials are not transparency and the researches on microstructure only after quenching, it is difficult to observe the dynamic microstructure evolution in real time. In this paper, the effect of liquid flow on the cellular and dendritic growth was investigated by the in-situ observation of SCN-1.8 wt% Ace transparent alloy during the directional solidification under the liquid flow. The cellular tip splitting is found in the presence of liquid flow and the cellular microstructure is smaller after the cellular tip splitting. The cellular spacing decreases as the flow rate becomes larger, but the spacing will become steady ultimately. At high growth rate the dendritic spacing increases with the increase of the flow rate, because the upstream side branches, which are accelerated by liquid flow, will suppress adjacent branches. But, at low growth rate the dendritic spacing decreases with the increase of the flow rate, because the lateral branches will exceed the tip of dendrite to form new dendrite by liquid flow.