Nanocrystalline Al doped nickel-cobalt ferrite [Ni_0.6Co_0.4Fe_2-xAl_xO₄ (x = 0.0, 0.1, 0.3, 0.5 and 0.7)] powders have been synthesized by sol-gel auto-ignition method and the effect of non-magnetic aluminum content on the nanosize particles and magnetic properties has been studied. The X-ray diffraction (XRD) revealed that the powders obtained are single phase with inverse spinel structure. The calculated grain size from XRD data have been verified using transmission electron microscopy (TEM). TEM photograph shows that the powders consist of nano meter sized grain. The size of nanoparticles decreases as the non magnetic Al content increases. Magnetic hysteresis loops were measured at room temperature with maximum applied magnetic field of 20 KOe. As aluminum content increases, the measured magnetic hysteresis curves became narrow and saturation magnetization (M_s), and coercive force (H_c) decreased. The reduction of magnetization with the increase of aluminum content is caused by non-magnetic Al³⁺ ions and weakened interaction between sub-lattices.