Abstract: Experimental test study was conducted on modeled reactor pressure vessel to investigate the steam-water counter-current flow limitation (CCFL) characteristics in the reactor downcomer during emergency core coolant (ECC) injection through the cold leg. In this study, the reactor pressure vessel of a PWR was scaled by 1∶5.8 using modified linear scaling method. The scaling focuses on the downcomer whose geometry is the same with the actual reactor. The core was simulated by steam generating tubes. ECC was injected through two cold legs and flowed out through a break in another cold leg. The mass flow rate of the inlet ECC, generated steam, penetrated water into the lower vessel, and bypass water and steam was measured. The inlet ECC flow rate was also scaled from actual reactor. ECC experiment data were obtained under different inlet ECC flow rates, subcooling degrees and core steam flow rates. During the experiment, the vessel pressure maintains between 0.2 MPa·abs and 0.6 MPa·abs, and the core steam temperature is within 10 ℃ superheat. Under each test run, the inlet ECC mass flow rate is unchanged, the core steam mass flow rate ranges from small to large corresponding to ECC total penetration to near zero penetration. The experiment results show that the steam condensation fraction increases with inlet water mass flow rate and decreases with core steam mass flow rate. The penetration water dimensionless velocity has a highly linear negative relation with the steam dimensionless velocity after onset of CCFL. CCFL model of Wallis and Kutateladze type was derived from the obtained experiment data. The effect of condensation on the steam flow rate in the downcomer was calculated through an empirical equation. Results show that the CCFL model trend for each inlet ECC mass flow rate is similar. Wallis and Kutateladze type model is capable for every inlet ECC mass flow rate. CCFL model parameter derived from dimensionless velocity has a highly exponential relation with the normalized inlet ECC velocity. In this way, the relations of steam dimensionless velocity and penetrated ECC dimensionless velocity under different inlet ECC mass flow rates could be calculated using one model. The analysis result shows that the mean uncertainties of Wallis and Kutateladze models are about 8.10% and 9.02% in this study.