A series of Al substituted yttrium iron garnet (Al-YIG) nanopowders with nominal formula of Y3Fe5 − x Al x O12 which x varied in steps of 0.0, 0.5, 1.0, 1.5 and 2.0 were prepared via mechanochemical processing. The samples were milled for 40 h in a high energy planetary mill and then calcined at different temperatures from 1,300 to 1,100°C. X-ray diffraction patterns reveal that the structure of nanopowders are bcc and the garnet phase has been obtained after calcining. The average crystallite sizes are in the range of 24–45 nm, using Scherrer’s formula. The lattice constant of the samples decreases by increasing Al concentration. To investigate the site preference of Al3 + ions, room temperature 57Fe Mössbauer spectra for the samples were recorded and analyzed. The hyperfine field values for octahedral and tetrahedral sites of the samples decrease by increasing x value. This decrease in magnetic hyperfine field reflects a reduction of the superexchange interaction by the progressive substitution of Al for Fe, which is also evident in the behavior of the magnetization and T C for the samples.