We report the nanoparticle Co0.2Zn0.8Fe2O4 spinel oxide, synthesized by room temperature mechanical milling. The system is stabilized mainly in spinel oxide phase after 78 hrs milling time and no other phases have been observed from the XRD spectra of the 100 hrs milled sample. We have studied the particle size effects on structural as well as on magnetic properties by annealing the 100 hrs as milled sample at different temperatures for 12 hours. The XRD and TEM data show that the particle size decreases with increasing milling time upto 60 hrs and then show an increasing trend with milling time. The particle size also increases with annealing the 100 hrs as milled sample with better crystalline structure. It is observed that the magnetic properties of the annealed samples can be correlated with the structural change without breaking the crystal symmetry of the cubic spinel phase of Co0.2Zn0.8Fe2O4 spinel oxide. The change in the crystal structural as revealed by the XRD spectra can be associated with the non-equilibrium to equilibrium cationic distribution between tetrahedral (A) and octahedral (B) sites of the spinel structure.