Some physical characteristics for describing the capacities of steam trap are defined in the present paper. Based on the analysis of the operating principle of disc steam trap, and the introduction of the physical characteristics, a mathematical model used to describe the steam leakage in steam trap is put forward. Then a formula used to calculate the amount of steam leakage is offered. The operating factors affecting the amount of steam leakage are theoretically investigated and verified. The experimental results are fully in agreement with the theoretical results. Based on these results, the authors put forward some basic claims and methods which are used to reduce the amount of steam leakage in the disc steam trap.