Abstract: A visualization study of the instantaneous flow patterns in a laminar junction flow is presented in the paper. The pulsed hydrogen bubble technique and video recording system allows the recording of instantaneous flow field in the present study. The arrangement of the hydrogen bubble wire was proven to be critical for revealing the fine structure of the junction flow. The junction flows produced by a rectangular block extending from the surface were tested with Reynolds number ranging from Reδ* = 2.74 × 102 to 3.19 × 102. The results illustrate the existence of primary horseshoe vortices and counter-rotating secondary vortices. Some new features, such as the behavior of the head portion of a primary HV in the formation region and its shoulders upstream of the block, were found in the visualization study.