A nongraphitizable carbon prepared from the cross-linked petroleum pitch and carbonized at 1473 K was found to have a unique structure and a charge capacity of more than 600 Ah/kg. A main peak of the 7Li Nuclear Magnetic Resonance spectra of the charged carbon shifted downfield with an increase of charge capacity. A Knight shift of lithium in the carbon charged to 600 Ah/kg reached 110 ppm when LiCl was used as the reference of 0 ppm. This shift was clearly distinguished from that of the lithium state in the first stage of the graphite intercalation compound, because the latter was observed at 45 ppm. A modified extended Hükel molecular orbital calculation showed that the average net electron density on lithium atoms drastically increased with increasing concentration of lithium atoms if the aromatic molecular planes are more than 0.5 nm apart. Both the experimental and theoretical results suggest that lithium atoms form clusters in this nongraphitizable carbon.