Abstract: Original RELAP5 is a non-real-time code, and has limitation on the data communication and calculation process control, so it is not very suitable for simulator. A new thermal-hydraulic system code for simulator, RELAPSIM, has been developed by employing RELAP5 at NSSE. The course of development consists of three major parts. Firstly, the time control function has been added into the code to meet the requirement of real-time calculation. Secondly, the function of controlled dynamic data communication was improved, thus thermal-hydraulic parameters can be exported from the code and can be input and modified by commands. Finally, the function of calculation process control was embedded in RELAP5 code, which provides the full ability to fulfill the multiple operations, such as starting up, shutting down, freezing, running, snapshoting, and resetting. This paper presents the development activities and the results of assessment by using RELAP5 and RELAPSIM.