Dam is a construction work to store water in hydropower and irrigation reservoirs. Statistical reports on 900 cases of dam failures in the world (except China) of the International Commission on Large Dams showed that 66% of the failures occurred in earthen dams, in which almost a half (46%) was due to internal erosion [1]. Although the monitoring systems were installed on the dams, from traditional techniques like using piezo tubes to modern techniques like using pressure, resistance, and temperature sensors, most of the leak cases were discovered by direct observation because the initial leakage occurred in relatively narrow range compared with the control of the monitoring network. When the percolation leakage is founded, besides monitoring the progress of seepage flow by the measurement of flowrate and sediment load, the parameters such as permeability or hydraulic conductivity of the leakage and infiltration progresses over time are actually requirements to help assess the impact of the leakage phenomenon to the safety of the dam. Tracer technique as a survey method in the field allows determining such important parameters as hydraulic conductivity and volume of the zone with leakage through the dam to characterize the hydrodynamics of the leakage. This paper presents the preliminary results of tracer technique applications to locate the leak point in the reservoir; to determine the transit time, permeability  velocity, hydraulic conductivity, and water saturated volume of leakage zone through the HT dam.