Atomic and electronic structures of grain boundaries in diamond have been calculated by using a transferable tight-binding method. The {122} Sigma=9 and {211} Sigma=3 tilt boundaries are dealt with based on atomic models, and results compared with the recent experiments in diamond and the calculations in Si. has been shown that grain boundaries in diamond have fairly large interfacial energies because of the large bond rigidity as compared with those in Si. Presence of three-coordinated sites at the interface has been discussed as compared with the observed electron energy-loss spectra from the Sigma=3 boundary.