Lithium intercalation of natural graphite and well-graphitized vapor-grown carbon fibers has been investigated by solid-state Li-7-NMR and by cyclic voltammetry. Chemical shift of Li-7 in Li-graphite intercalation compounds (Li-GICs) of natural graphite occurs in two regions, >40 and <10 ppm. The former shifts were assigned to GICs with the in-plane density of LIC6 and the latter shifts were assigned to those with the in-plane density of LiC9. Phase diagram of Li-GICs during electrochemical intercalation is discussed. In comparison to natural graphite, Li-7 in the vapor-grown carbon fibers showed higher chemical shifts. The passivating film was also examined by Li-7-NMR and structural change of the film during the first charge-discharge cycle is indicated. Scanning electron microscopy imaging of the passivation film supports the Li-7 NMR data during charge-discharge. The swelling and shrinking of passivation is evident by scanning elecron microscope image and nuclear magnetic resonance data. The thickness of the solid electrolyte interface layer depends on the nature of salt, solvent, carbon origin, and current density.