In this paper, aided by the in situ transmission electron microscopy, the dynamic microstructural evolution of CuO nanowires during sodiation process were directly monitored in a solid cell. As evidenced by the in situ observation, the sodiation procedure is controlled by the long-ranged diffusion transport mechanism. Consistently, according to the sodiated products, it was found that the sodiation process of CuO could be divided into three steps. Furthermore, compared with the results of previous ex situ experiment, two extra phases: NaCuO and Na6Cu2O6 were observed. It is speculated that the discrepancy may originate from the different reaction environment and types of electrolytes. Our results facilitate the fundamental understanding of the sodiation mechanism of CuO and have implications for better designing of the electrodes in sodium ion batteries.