Various types of soil physical properties are affected by texture and structure. Our objective was to determine aggregate structure effect on the soil dielectric property of an Andisol measured by time domain reflectometry (TDR). The relationships between volumetric water content (θ) and dielectric permittivity (ε) for both a wet-sieved aggregate and its crushed sample were examined and compared. In the θ–ε relationship for 0.1- to 2.0-mm-diam. wet-sieved aggregates, the gradient of the θ–ε curve moderately changed at a volumetric water content (critical water content), although this property disappeared when we crushed the aggregate structure. Furthermore, the critical value corresponded to the water content of the plateau in a bimodal-type water retention curve. We suggest that effects of aggregate structure on a soil's dielectric property are involved in the aggregate sizes, the configuration of water in aggregates, the processes of water filling in intra- and interaggregate pores, and the low ε value of bound water adsorbed on soil surfaces.