Generally, Si multicrystals, which are grown by a casting method using a crucible, contain many grain boundaries (GBs) and crystal grains with various orientations. Since the grain size has increased as a result of improving in the growth technique, instead of GBs, subgrain boundaries (sub-GBs) have become major defects acting as recombination centers for photogenerated carriers. In this chapter, the study of sub-GBs in Si multicrystals is comprehensively reviewed with the authors’ current results.