In proton exchange membrane electrolyzer cells (PEMECs), the oxygen generated at the anode side by electrochemical reactions is forced out of cells through the micro channels and liquid/gas diffusion layer, meanwhile, the water flows through the same pathways to reach the reaction sites in an opposite direction, during this process, the complex two-phase counter flow is formed. The two-phase transport conditions strongly affect the PEMEC performance. In this study, a long-working-distance, high-speed, and micro-scale visualization system coupled with a specific designed transparent PEMEC are introduced to investigate the two-phase flow in situ. Optical data can be used to aid in understanding the effects of two-phase flow on the performance of PEMECs. The two-phase flow phenomena in horizontal micro channels of an operating PEMEC is revealed in this study. In addition, the dynamics of gas bubble evolution are also identified. More quantified results will be studied in the future.