Cobalt ferrite nanoparticles with different crystallite sizes have been synthesized using the coprecipitation method. In this work, the structure and phase purity was analyzed by using X-Ray diffraction (XRD). The compound analysis was done using the Scanning Electron Microscope (SEM) with Energy Dispersive X-ray analysis (EDAX). The magnetic properties were studied using the vibrating sample magnetometer (VSM). The magnetic crystalline anisotropy constant of CoFe2O4 was calculated using Law of Approach to Saturation (LAS) model. This plays a major role in the hyperthermia studies. © 2015 Scientific Publishers. All rights reserved.