The effects of the structures on the energy gaps in luminescent carbon nanoparticles (CNPs) were investigated. On the one hand, we fabricated CNPs with the different structures and a uniform size and then analyzed their photoluminescence (PL) behaviors. On the other hand, we calculated the dependence of the structures on the energy gaps in CNPs by a simple quantitative model. Both the experimental and calculated results show that the luminescent CNPs contain a mixture of sp² and sp³ bonding and hence their PL behaviors and energy gaps are determined by the fraction of sp²-hybridized carbon atoms.