Studied different size nano-magnetic particles to understand the change in the physical properties because large volume fraction of atoms occupy the grain boundary area. Synthesized nano particles were annealed in varying temperature. Crystallinity improves on increasing temperature 373K to 1173K for 2 hrs duration. The material remains single crystalline phase in all the temperature as shown in (Fig. 5.5). At the applied magnetic field of 5000 Oe observed from the magnetization curves is found to increases from 18emu/gm for the sample annealed at 373 K to 74 emu/gm for the sample annealed at 1173 K. Also the corecivity first increases from 0 Oe for the particles annealed at 373K up to a maximum value of 1200 Oe for the particles annealed at 973K, on further annealing the particles at 1173K it is observed that the corecivity decreases to 1000 Oe. The variation of corecivity with particle size is also explained on the basic of domain structure, diameter of particle and crystal anisotropy. The saturation magnetization for the nanocrystalline cobalt ferrite is found lower to their bulk value, which can be attributed to the surface spin canting. Further, the size reduction of magnetic particles leads to several unusual properties like disorder of surface spin (spin canting), surface anisotropy, SP nature and hence it can be tailor for specific applications.