Mn-Zn ferrite nanoparticles were fabricated via conventional ball-milling and their magnetic properties were investigated. By ball-milling of Mn0.53Zn0.42Fe2.05O4 agglomerates for 48h with and without a dispersant (Darvan-C (ammonium polymethacrylate)), nanoparticles having average particle size of 60 nm were obtained. The saturation magnetizations (Ms) of thus obtained Mn-Zn ferrite nanoparticles were 49 and 62 emu/g for dispersant-added and dispersant-non-added one, respectively. When the nanoparticles were heat-treated at 400, however, the Ms became comparable: 63 and 65 emu/g. When the nanoparticles were heat-treated at 600, moreover, the Ms became comparable with that of bulk ferrites: 75~78 emu/g. These magnetic properties were attributed to the surface spin disorder effects resulting from the coating of organic dispersant molecules on the surfaces of the nanoparticles as well the structural disorder on the surfaces the nanoparticles.