The spectral properties of nanoparticles Gd2O3:Eu3+ have been investigated by using Judd Ofelt theory at 77K. Intensity parameters Ωλ(λ＝2，4) were calculated using the experimental data of 5D0→7F2 and 5D0→7F4 transitions by taking the magnetic dipole transition 5D0→7F1 as reference. The intensity of the transition is almost completely unaffected by the local environment. It is indicated that intensity parameter Ωλ changes with the variation of diameter of nanoparticles Gd2O3:Eu3+,dramatically different from that of the bulk material.With decreasing diameter of the nanoparticles Gd2O3:Eu3+ from 135 to 15nm,the quantum efficiency of the emitting level 5D0 reduces from 23.6% to 4.6%. The smaller the diameter of nanoparticles Gd2O3:Eu3+, the bigger the ratio of surface to volume and the more the surface defect.It is the enhancing of the non radiation channels that results in the decrease of the quantum efficiency. The other two non radiation channels, charge (Eu3+-O2-) transfer state and multiphonon nonradiative re excitaton, were also studied.