An in situ X-ray photoelectron emission investigation revealed that the size trend of the 2p(3/2) binding-energy shift (BES) of Ni nanoclusters grown on SiO2 substrate follows the prediction of the bond order-length-strength (BOLS) correlation theory [30]. Theoretical reproduction of the measurements turns out that the 2p(3/2) binding energy of an isolated Ni atom is 850.51 eV and its intrinsic bulk shift is 2.70eV. Findings confirmed that the skin-depth local strain and potential well quantum trapping induced by the shorter and stronger bonds between under-coordinated surface atoms provide perturbation to the Hamiltonian and hence dominate the size dependent BES. (C) 2010 Elsevier B. V. All rights reserved.