The effect of the grain boundaries (GBs) in a Cu foil on the CVD growth of graphene was investigated via radiation-mode optical microscopy, which we developed for the real-time observation of growing graphene domains. Although the GBs had the typical width and depth of a few micrometers and hundred nanometers, respectively, they had little effect on the nucleation and the lateral growth of the graphene domains. However, increasing the growth time caused the diffusion of C atoms from the inside of the Cu foil, which was likely to result in additive nucleation and the enhancement of the growth speed at the GBs. (c) 2017 The Japan Society of Applied Physics