The f-d transition of Ce3+and Tb3+in BaBPO5 was studied theoretically using the parametric Hamiltonian model.In order to overcome the difficulty in determining many of the parameter values,we adopted the model-space effective Hamiltonian method to determine the crystal-field parameters and spin-orbit parameters values.The method made use of the energies and eigenvectors,which were obtained from an ab initio calculation using the relativistic self-consistent discrete variational Slater software package(DV-Xα).Other parameters,which were less dependent on host crystals,were taken from published data.The calculated values of parameters were reasonable,and the energy-levels and f-d transition spectra agreed reasonably well with the measured excitation spectra of 5d-4f emission.