We investigated the electron affinity on hydrogen-terminated nano- (NCD) and ultranano-crystalline diamond (UNCD) films to reveal influences of defects, sp2 and grain boundaries on photoemission properties. To compare properties, single crystalline IIa diamond, optical grade poly-, nano-, ultranano-crystalline diamond films, a diamond-like carbon film, a highly oriented pyrolytic graphite have been characterized. All hydrogen terminated SCD, PCD and NCD samples show continuous sub-band yield, arising at about 4.4 eV, regardless of their crystal and/or grain sizes, while UNCD, DLC, and HOPG do not show the specific photoemission in the regime hν < 6 eV. Note that the UNCD film grown with some amount of hydrogen in the gas phase (UNCD-H) shows a comparable yield spectrum as hydrogen-terminated SCD. According to the normalized spectra, obviously, the electron emission mechanism on hydrogen-terminated NCD films is the same as that of SCD with a NEA of -1.1 eV. The sub-band yield from NCD is even larger than that of SCD, which is attributed to surface enlargement.