Monophase nanocrystalline powders belonging to the La 1-x Sr x MnO 3±δ system (0≤x≤1) with a perovskite structure have been obtained by mechanochemistry synthesis using a planetary ball milling equipment from La 2 O 3 , SrO, and Mn 2 O 3 mixtures. The solid state reaction was complete after one hour of milling treatment. For all the compositional range, the diffraction domain was very small and the structure appeared as a pseudo cubic perovskite. After annealing at 1100°C under static air, the symmetry evolution due to the La substitution by Sr was analyzed by x-ray and electron diffraction. Samples with x = 0, 0.25, 0.5, and 0.75 were assigned to R-3c space group (167) in the rhombohedral system and perovskite structure. However, the symmetry of the last term of the system (x=1), SrMnO 3±δ sample, changed to P6 3 /mmc space group (194) in the hexagonal system. The terms with x = 0.8, 0.85, and 0.9 presented mainly rhombohedral symmetry.