BaTiO₃ (BT) is a typical ferroelectric material used in capacitors. For miniaturization of devices, nanoscale BT research has been actively conducted, along with research on the enhanced dielectric properties of nano-capacitor materials. The ferroelectricity of BT at room temperature is strongly related to its tetragonal crystal structure. It has been reported that the dielectric constant increases as BT particle size decreases. However, below a certain critical size, ferroelectricity of BT disappears and it exhibits characteristics of a cubic crystal structure at room temperature. Various measurement methods have been deployed to study the relationship between particle size and structure and the compound’s ferroelectricity. Among them, small angle / ultra-small angle X-ray scattering has been used to investigate the particle size of powder materials non-destructively, without dilution. Here, the average particle size and the size distribution of a BT powder are evaluated by ultra-small angle X-ray scattering.