Al substituted mixed ferrites of Zn0.3Mg0.7AlxFe(2-x)O4 (x=0.0, 0.1, 0.2, 0.3, 0.4) prepared by sol-gel auto combustion method. The structural studies using XRD and SEM reveal that the obtained samples were in cubic structure with single phase. As the Al substitution increases in this ferrite system the lattice constant decreases from 8.435 Å to 8.362 Å. The average crystallite size calculated using Debye-Scherrer equation and is in the range 26 to 92 nm. The variation in lattice constant and crystallite size attribute due to the replacement of Fe3+ by smaller radii of the Al3+ ion. The magnetic measurements using Vibration Sample Magnetometer (VSM) indicates that the Al3+ preferably occupy B-sites rather than A site. FTIR spectra exhibited two prominent absorption bands in ν1 and ν2, which confirms the redistribution of ions among tetrahedral and octahedral site and spinel nature of these ferrites.