Abstract: During CAP1400 small break loss of coolant accident, all passive core cooling equipments will put into operation, and present complex and unique phenomena, in order to validate the design and the computer code, and to understand the key phenomena, CAP1400 integral passive core cooling test was performed. The main characteristics of integral facility are 1∶3 scaled height, maximum operation pressure of 9.2 MPa, simulation with equal pressure. In the tests, the phenomena of cold leg stratification, accumulator nitrogen injection, and downcomer asymmetric temperature distribution are found, and no core uncover in small break loss of coolant accident is testified. Finally the validation of the modified RELAP5/MOD3 with the test results was provided.