The STAR-LT code is developed for tracking the two-phase water level through the modification of the STAR code. The level tracking logic can be utilized effectively when there exists a definite phase separation boundary. The logic is developed on the basis of the two-region two-fluid model. In order to describe the transportation of vapor through the interface of the two-phase mixture region and vapor region, a bubble rise model is used. Since the level can be treated as a system variable in STAR-LT, coarse nodalization can be used to describe the system. In the present sample calculation of the blowdown of the tank, only one node is used. In order to verify the level tracking logic in case of depressurization, the GE tests and the Marviken T-11 test are simulated by STAR-LT. And to verify the code for a vessel with heat addition, the EPRI 336-rod bundle experiment is simulaed. It is shown that the simulated results are in good agreement with the experimental data. The influence of drift velocity on the two-phase mixture level is evaluated using various drift correlations. The Ishii correlation is found to be the best one for the present tank blowdown simulation.