Cast polycrystalline silicon for solar cell contains mostly straight twin boundaries which are thought to have little effect on the electrical activity. There are, however, some complicated grain boundaries in it. One of these boundaries consists of slightly curved and straight parts. The structure of this boundary was analyzed to investigate the difference of these two types of boundaries. The conventional transmission electron microscopy (TEM) found that this slightly curved boundary was the zigzag shaped boundary made by (112) and (211) planes. High resolution electron microscopy (HREM) confirmed that (112) plane was the boundary of {112} Sigma 3 twin boundary which formed a straight grain boundary at the other end of the analyzed grain boundary, and also confirmed that (211) plane was also the boundary of {112} Sigma 3 twin boundary which intersected with the former twin boundary at an angle of 120 [deg].