Containerless processing was used to investigate the glass-forming behavior of Al2O3-Y2O3 glass. The amorphous bulk samples were obtained at compositions with 25-37.5 mol% yttria when the melt was cooled at a cooling rate of similar to250 K/s. Although small spherical particles (similar to10 mum) with the same composition of the matrix were detected in the amorphous samples with 32.5-37.5 mol% yttria, the microfocus X-ray diffraction result indicated that the small spherical particles were crystalline Y3Al5O12 garnet (YAG), rather than being amorphous. This observation suggested that small YAG particles could not grow larger after their nucleation, because of the high viscosity at high undercooling and the high cooling rate, which would graze the nose of the continuous cooling temperature diagram of YAG.