This article reports a novel fabrication method for In(OH)3 from indium oxalate by hydrothermal process. Hydrothermal decomposition of indium oxalate at 180 degrees C for 10 h results in In(OH)3. The influence of hydrothermal experimental conditions such as temperature, time on the formation of indium hydroxide was investigated. The self-assembly process was strongly influenced the experimental conditions. The thermal decomposition of In(OH)3 at 400 degrees C results In2O3. The synthesized In(OH)3 and In2O3 were characterized using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), high resolution transmission electron microscopy (HR-TEM), thermal analysis (TGA and DTA), diffuse reflectance spectra (DRS), and nitrogen adsorption analysis. The XRD patterns indicated the formation of well crystallized cubic phase In(OH)3 and In2O3. The FE-SEM results indicated formation of In(OH)3 and porous In2O3 nano/micro-cubes. The photocatalytic activity of the synthesized In(OH)3 was studied under UV light irradiation and results showed that the In(OH)3 photocatalyst was efficient for dye degradation. We proposed a plausible mechanism for the formation of In(OH)3, and In2O3 self-assembly.