The early stage of phase separation in aluminoborosilicate glass for liquid crystal display substrates was investigated using a scanning electron microscope (SEM) and small-angle X-ray scattering measurement device (SAXS). The phase-separated particles were detected with SAXS in samples in which no particle was observed with SEM. The particle diameter in an as-formed sample was 2 nm. The relationship between the diameter and heat-treatment time indicates that the phase separation proceeds by a diffusion-controlled process. Based on the results of Fourier-transform infrared spectroscopy, fast compositional change may occur in the early stage of phase separation.