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A series of diamond nanoparticles with different surface terminations, (I) CH_n (n = 1, 2) and (II) CH_n (n = 1–3) species, have been investigated by means of density functional theory (DFT) to probe the effects of the terminated CH_n species on the geometric and electronic structures and related properties. Our results show that quantum confinement effects of HOMO–LUMO gap occurs for the series I particles with size up to 1.1 nm, in contrast to the much weaker decay of the gap for larger size. With the existence of additional CH₃ species, for size larger than 1 nm, the series II particles have larger gaps than the series I counterparts, which may be even larger than that of bulk diamond. The compositions of HOMO and LUMO are responsible for the different behaviors in the quantum confinement, which agrees with the experimentally observed spectral feature in the X-ray absorption measurement. In addition, our results show that the negative electron affinity is strongly dependent on the C/H ratio for the hydrogenated diamond nanoparticles.