Abstract: For the safety analysis of pressurized water reactors, it is necessary to accurately predict the gas-liquid flow relationship under the gas-liquid countercurrent limitation (CCFL) conditions. In this study, with the use of the experimental method of submerged exhaust, the CCFL characteristics of the vertical pipes with different pipe diameters and an identical pipe length were tested. Then, the CCFL correlation models for vertical pipes were analyzed. The main conclusions are: ①The flow pattern in vertical pipes is annular flow under the CCFL condition. When the superficial gas velocity is large, the liquid film in the large diameter pipe shows a churn-like motion, while the liquid film in the small diameter pipe behaves a stable falling flow with wavy interface. As the superficial gas velocity decreases, they both convert to a falling film flow with smooth interface. ②The model based on Wallis numbers presents an over-correlation to the effect of pipe diameters on the CCFL of vertical pipes; meanwhile, the models based on the Kutateladze number or the Froude-Ohnesorge number also fail to correlate well for the diameter effect on the CCFL of vertical pipes. ③A new dimensionless parameter is proposed to depict the CCFL of vertical pipes, which can correlate the diameter effect satisfactorily and can indicate the effect of fluid properties on the CCFL of vertical pipes simultaneously.