Diamonds grown by chemical vapor deposition and high-pressure high-temperature processes were mass analyzed by a newly developed scanning atom probe. The mass analysis of the diamonds revealed that the diamonds contain a large amount of hydrogen not only in the surface layer but also in the deep underlying layers and that carbon atoms form the clusters, C-3, C-5, C-8, and C-16 The clustering carbon atoms could be strongly bound in the diamond structure. The detection of a large number of neutral particles indicates that most cluster ions were decomposed into a positive ion and a neutral particle immediately after field evaporation. The interesting finding is that most carbon atoms are field ionized as multi-ionized ions such as C2+, C3+, and C4+ and/or CH2+, CH3+, and CH4+ with the mass peak of H+, while the major mass peaks are C+ and/or CH+ when no hydrogen peak is found. This may suggest that the binding state of carbon atoms in the diamonds varies with the binding state with hydrogen atoms.