The sinterability of high-purity, nanocrystalline Y(2)O(3) without any additives was investigated by spark plasma sintering (SPS) for a combination of low sintering temperatures (850 degrees-1050 degrees C) and low heating rates (2-50 degrees C/min). At a sintering temperature of 950 degrees C and a heating rate of 2 degrees C/min, the SPS yielded a polycrystalline Y(2)O(3) having a relative density of 99% and an average grain size of 190 nm. The Y(2)O(3) bodies sintered at 950 degrees and 1050 degrees C for 1h at the heating rate of 2 degrees C/min exhibited an in-line transmittance of 6%-46% in a wavelength range of 400-800 nm. A high-resolution transmission electron microscopy observation and chemical analysis by an energy-dispersive X-ray spectrometer revealed that the sintered Y(2)O(3) bodies were single-phase materials without any grain-boundary amorphous layer or impurity contamination. An isothermal SPS experiment indicated that the grain-boundary mobility is significantly enhanced by the SPS. An electron energy loss spectrometry indicated that SPS changed atomic configuration of the grain boundaries. The improved sinterability of Y(2)O(3) is attributed to the enhanced diffusion that arises from defect reactions activated by the SPS.