The double-hexagonal close-packed localized moment 5f system UPd3exhibits four phase transitions below 8 K. We present newmeasurements of the magnetic susceptibility of UPd3single crystals. The phase transitions are attributed to a sequenceof antiferroquadrupolar ordered structures of the U 5f electrons onthe quasi-cubic sites.From a comprehensive analysis of the neutronscattering, x-ray scattering, and bulk property measurements of thissystem, we have deduced a new crystal field model for UPd3.This model, which has a doublet ground state on the quasi-cubic sites with a firstexcited singlet some 4 meV above it, provides a qualitative understanding of thesuccession of quadrupolar phase transitions and order parameters of UPd3.Moreover, the new model also explains the excitationsobserved, by inelastic neutron scattering, in UPd3at 2 K.