A nanofluid is a suspension in which nanoparticles are mixed with a base fluid and is known to have higher thermal conductivity than conventional working fluid. The easiest way to increase the thermal conductivity of a nanofluid is to increase its concentration. However, as the concentration of nanofluid increases, sedimentation stability decreases due to the tendency of particles to aggregate. In the case of nanofluids with low sedimentation stability, the aggregated and precipitated nanoparticles interfere with the flow of the fluid, thereby reducing the heat transfer performance of the thermal system. Sedimentation stability is essential for commercializing nanofluid and its use in the industry. Currently, methods for evaluating the sedimentation stability of nanofluids include dynamic light scattering (DLS), Zeta potential analysis, and visual observation utilizing TEM and SEM. These techniques can be applied to specific concentration ranges, types of particles and fluids, and particle sizes. In this study, we propose a method for measuring nanofluids' particle size, concentration, and sedimentation stability by using the 3ω method. The temperature amplitude of the 3ω heater, depending on the increasing thickness of the particle sediment layer of nanoparticles falling at a constant speed, was analyzed. A mixture of Al₂O₃ nanoparticles and de-ionized water were measured and the distribution of nanoparticle size obtained from a DLS method was found show good agreement with the 3ω measurement result.