High-resolution electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS) were performed on grain boundaries of an artificial diamond film to examine the correlation between their atomic and electronic structures. Characteristic grain boundary structures that were different from the other covalent bonding materials were shown in Sigma 3 CLS and Sigma 9 CLS boundaries: (112) Sigma 3 boundaries were symmetrical in atomic arrangement and Sigma 9 boundaries were parallel to the (114) plane, although 'dangling' bonds were supposed to be present in it. A new line that corresponds to the pi* state was found in addition to a major sigma* line in the EELS spectra of both (112) Sigma 3 and (114) Sigma 9 boundaries. The new pi* line suggests that there is pi bonding in both boundaries instead of the dangling bond. Reconstruction of a dangling bond into the pi state may stabilize the diamond grain boundary.