We evaluated the properties of crystalline defects in silicon substrate, and clarified the origin of small-angle grain boundaries. In order to eliminate the effects of grain boundaries, the ingot was fabricated by unidirectional solidification technique with seed crystal. In single-crystalline region, σ3 twin boundaries and SiC precipitates were observed near the seed crystal. No obvious correlation between twin boundaries and precipitates was observed. These defects decreased once and the precipitations appeared again. The density of precipitates increased through the crystal growth procedure. These precipitates were consisted of Si, C, and N. After the precipitation density increased, the small-angle grain boundaries appeared and some precipitates were observed at the boundaries. We considered the precipitation consisted of light element impurities such as C and N was one of the major origins of the small-angle grain boundary generation.