▎ 摘　　要

Grains and grain boundaries (GBs) in graphene are vital for the control of its properties; however, engineering or controlling them by growth remains a great challenge. Here we discover that the dynamic formation of GBs within chemical vapor deposited polygonal graphene flakes is described by a geometric rule. A GB is formed to be symmetrically tilted and a continuous straight line, and the key parameters including end point, direction of GB line, and misorientation angles between adjacent graphene grains can be determined solely by the geometries of the polygonal graphene flakes. We also show the growth control over the length of straight graphene GB lines and demonstrate the capability of parallel fabrication of field-effect transistor devices across predicted GBs in a straightforward manner. This work constitutes a significant step forward in engineering grains and GBs in graphene.