Abstract: External reactor vessel cooling (ERVC) is one of the important severe accident mitigation strategies to achieve in-vessel retention(IVR) of melt core debris under severe accident conditions. Referring to the IVR-ERVC conditions for the prototypical pressure vessel lower head wall of elliptic-shaped, a critical heat flux (CHF) test campaign was, in the paper, carried out upon a full-sized thick test block section which was installed in a one-dimensional full height natural circulation test loop. Eighteen groups of heating rods with independent power control were inserted into the test block. Eight experimental measuring points were evenly distributed on the heating wall of the test block along the inclination angle, and the heating power shapes of each experimental measuring point were determined according to the Theofanous’ power shaping principle. Thermocouples were arranged near the heating wall and on all sides of the test block to obtain the temperature information during heating and CHF occurring. CHF data as well as their distribution along ellipticalshaped outer wall of test block were obtained. Meanwhile, preliminary evidence of typical CHF triggering mechanism on downwardfacing curved heating wall was deduced through the visual observations during the test. The visual observations show that when the evaporative drying area of the liquid film under the vapor block is large enough, it is difficult to cool the heating wall of test block. The wall temperature rises rapidly, and CHF occurs. Furthermore, effects of inlet subcooling, flooding water level, flow resistance and natural circulation flow rate, as well as the gap size of ERVC channels on CHF limits are experimentally studied. Test results show that, CHF increases with the increase of the inclination angle of heating wall, the increase of inlet subcooling can significantly increase CHF. Increasing the inlet subcooling can reduce the liquid temperature in the twophase boundary layer and effectively delay the evaporation of the liquid film, so as to improve the CHF. In the base cases and inlet subcooling cases, the relative decrease of CHF occurs in the uppermost section of the heating wall, which is called “exit phenomenon”. The CHF of the heating wall increases slightly with the increase of liquid level. While the change of natural circulation flow resistance and flow rate in a certain range has a rather limited impact on CHF. According to the CHF triggering mechanism, the flow rate change is not large enough to cause the instability and fracture of vapor block and the near wall flow structure does not change significantly, so the impact is limited. The influence of the change of gap size of ERVC channel on CHF is quite complicated. It seems that the relative relationship between the gap size and the thickness of twophase boundary layer, as well as the streamline constraints of the flow channel wall on the vapor phase both have influence on the CHF quantity and distribution.