Leak-before-break analysis of pressurized vessels and pipes is an important technique to guarantee the safety of reactor pressurized components. This analytical method involves many technical problems that need intensive study. Among these problems, the leak rate of the coolant is far from being well understood. The results of the current leak-rate models may have large errors. Little study has been carried out on the leak rate of gas coolant used in a high-temperature gas-cooled reactor (HTGR). This paper introduces experimental and theoretical studies on the leak rate of a gas medium. The focus is on the results of the experimental studies. A series of experiments for the leak rate of a gas medium were conducted based on preliminary simulation of the penetrated crack. Systematic data were obtained. The laws of a gas medium flowing through a narrow slit are summarized.