The role of thermal treatment at different time and temperature on the structure of Yttrium Aluminium Garnet nanopowders doped with europium ions (Eu:YAG) will be presented in order to understand the structure evolution. Nanopowder of Eu:YAG has been obtained by co-precipitation method treating aluminum, yttrium and europium nitrates solution with ammonia. The hydroxides were calcined at different temperature (900, 950 and 1050°C) for different times (1, 2 and 6 hours). The presence of Eu was confirmed by Energy Dispersive X-ray analysis. By X-Ray Diffraction has been found that the sample treated at 900°C for 1h is constituted essentially by YAG phase, but a minor hexagonal YAlO3 phase is present also. At longer time treatment and higher temperature, the samples are constituted by only YAG phase. Cubic garnet lattice parameters increase with both temperature and time. For the sample treated at 900° for 1h, Electron Microscopy showed spherical particles agglomerates whose size is smaller than 40 nm. Particles sintering has been observed at higher temperature and longer time.