In colloidal quantum dots (QDs), the intrinsic weak bonding ability as well as the steric hindrance of the organic ligands inevitably produces a large number of dangling bonds (DBs) on the surface atoms. Such DBs can lead to the reduction in the luminescence efficiency by generating the trap sites for charge carriers, but their detailed behaviors are still unclear. Here, we investigated the fundamental features of the DBs on the surface of InP QDs through density functional calculations supported with experimental results. For InP core, both In- and P-DBs create the size-independent DB energy levels with the localized electron densities acting as the trap site. Also, it is expected that the trap emission noticeably observed at 10 K is originated in In-DB rather than P-DB. The passivation of the ZnS shell brought dramatic changes in the optical properties of DBs even when the shell covered only part of the InP surface. The partial ZnS coating pushes the energy levels of the In- and P-DBs to the near position of band edges, and furthermore changes the electron densities of the levels to be more delocalized. Such itinerant distribution of DB-orbitals in InP/ZnS QD significantly improves their optical intensities comparable to that of the band-edge transition, which is validated by the absorption calculations and the luminescence measurements.