Abstract: In traditional pressurized water reactor, the heat flux of hot section in U-tube of steam generator is larger than that of cold section. This varying heat flux is the cause of inconsistency in boiling features of the coolant. Due to the large size and complicated structure of real steam generator, the actual internal flow characteristics are still unknown. To get the deep understanding of the flow in secondary side of steam generator, a visual test facility was established which consisted of only one row of non-uniform heating tubes. In the wake of boiling phenomenon, a high speed camera was used to get the insight of the bubble flow. The bubble’s transverse velocity was gotten by using the pictures. The results show that the dissimilarity in heat flux on hot and cold sections results in greater deviation of void fraction, and strong transverse currents are produced in U-bend section as a result of natural convection.