Abstract: Based on the particle image velocimetry (PIV) technique, the flow visualization in the fully developed 5×5 rod bundle channel at low Reynolds number was studied. The Reynolds number ranges from 310 to 12296. The experimental results show that the relative velocity gradient in the rod bundle channel is more significant at low Reynolds number. With the increasing of Reynolds number, the velocity distribution in the rod bundle channel tends to be uniform. The transition observed by the resistance characteristics in the rod bundle channel are more ambiguous than those in the circular tube, and the transition Reynolds number is about 900. Under the influence of low Reynolds number effect, the dimensionless RMS velocity decreases with the increasing of Reynolds number, while near the transition Reynolds number, the dimensionless RMS increases with the increasing of Reynolds number. Besides, the experimental data can be used to verify the applicability of the turbulence model to different Reynolds Numbers.