Cerium doped lutetium aluminium garnet (Lu3Al5O12:Ce, LuAG:Ce) displays particularly promising performance and has a great potential to be used in PET and SPECT. LuAG:Ce phosphors was synthesized with a simple sol-gel process. The structure was identified with XRD. Variations of luminescent properties and defects of LuAG:Ce phosphors with different heat-treatment temperatures and Ce3+-dope concentrations have been investigated, with the aid of emission spectra and positron annihilation lifetime measurements. The influence of defects on the luminescence properties was discussed.