Abstract: In this study the flow pattern and heat exchange coefficient the have been obtained,and relation between them have been determined in the circumstance of vapour lquid two phase boiling of water coolant through a even heated straight tube.The influences of the coolant flow parameters on the flow pattern and heat exchange have been studied by changing the coolant flow speed,or the flowrate at the entrance under certain heating conditions.As a result,the best flow pattern for the heat exchange coefficient in which the annular flow zone is the longest without “burn down”.Then the empirical formulas have been found for designing.The results of numerical calculation are in good agreement with the data of tests.