In the case of loss of the residual heat removal system under mid-loop operation during shutdown of the pressurized water reactor (PWR) plant, steam generated in a reactor core and water condensed in a steam generator (SG) form a countercurrent flow in a hot leg. In this study, to improve a countercurrent flow model in a transient analysis code, experiments were conducted using a scale-down model of the PWR hot leg. Flow patterns and countercurrent flow limitation (CCFL) characteristics were measured. A rectangular duct, whose height was about 1/5 of the hot leg diameter, was used to simulate the hot leg. Air and water at atmospheric pressure and room temperature were used for the gas and liquid phases. Water levels in the hot leg were also measured to investigate void fraction distributions. CCFL characteristics were well correlated with the Wallis-type correlation, and agreed well with the flow pattern transition from wavy to wavy-mist flows.