An experimental study for alternative ECCSs for a PWR was performed with the ROSA-II facility. It was found through the tests that the combined injection of hot water into the upper plenum and cold water into the lower plenum accompanied by a low pressure coolant injection system in the hot legs is quite effective for core cooling through the whole period of a LOCA in the case of a cold leg break. The test results were compared with analytical results of the RELAP4J code. The code is found capable of estimating discharge flow behavior fairly well and can predict the overall fluid behavior in the tested method of the improved ECCSs. However, the calculated core heat transfer disagrees with the test data when the counter-cuurent flow of the two phases on the core is dominant.