Partitioning Interwell Tracer Test (PITT) developed in the petroleum industry is a useful method to quantify the volume of Non-aqueous phase liquid (NAPL) in contaminated aquifer. PITT provides us information about the characterization of the NAPL volume and distribution in relatively large scale area. When we perform PITT, we use the basic assumption that tracers are partitioned to NAPL completely during PITT. However, the assumption is not verified yet. Under practical conditions, we have to be careful of (1)loss of NAPL during PITT, (2) NAPL migration process, (3) sorption of NAPL to organic materials or minerals in aquifer, (4) in-homogeneous tracer partitioning to NAPL because of the heterogeneous NAPL distribution and form, (5) accuracy of measuring tracer partitioning coefficients. The purposes of the study are (1) to estimate the unknown quantity of NAPL placed in a column filled with glass beads and water, and in a pipette filled with water, and (2) to analyze the accuracies of these estimations by PITT method. As a result, the ratios of estimated NAPL volumes and actual NAPL volumes ranged from 0.59〜1.4. The reliability of NAPL estimation by means of PITT decreased when the flow rates of tracers were too low.