Abstract: Based on system code RELAP5 and computational fluid dynamics code CFX, the RELAP5/CFX coupled code wasdeveloped by means of parallel virtual machines technique and CFX USER FORTRAN. Under the single phase state, verification of coupling between the two codes was performed with horizontal pipe blowdown problem firstly. Then a double T-junction mixing experiment was simulated. Compared with RELAP5 stand-alone calculations, the coupled code reveals three-dimensional mixing phenomena more clearly. Through more researches and development, the coupled code willbe used for mixingproblemsin the nuclear reactor safety analysis.