Ultra fine Mn0.2Zn0.8Fe2O4 magnetic particles are developed by high energy ball milling technique and investigated for physical properties. The crystalline phase, crystallite size, surface morphology, metal oxide bonding and porosity of these magnetic particles are analyzed. The porosity increases on increasing the grinding period due to decrease in the particle size and crystallinity reduces. The IR spectra measured in the range of 4000-400 cm-1 exhibit symmetric stretching mode of (FeO4) and (ZnO4) tetrahedral at 669.7 cm-1 and 545.6 cm-1. The lattice strain induced by ball milling process has been determined. The porosity plays an important role in chemisorption and physisorption of species on the sites of the particles. This property of these particles has been exploited for the applications of humidity sensor.