The transition from incomplete to complete grain boundary (GB) wetting has been studied in the (Cu) + L region of the Cu-In phase diagram in the broad interval of In concentrations from 4 to 22 wt% In. The number of completely wetted GBs increases with increasing temperature and In content. With increasing amount of the melt the GBs with non-zero contact angle become apparently completely wetted. A geometrical model describing the influence of the amount of the liquid phase on the mean contact angle and the portion of completely wetted GBs has been developed. Using this model, the values of the temperatures of the start T (wmin) = 715 A degrees C and the end of the GB wetting transition T (wmax) = 986 A degrees C in the Cu-In system were determined.