Present study was conducted to study the in vivo biocompatibility of cobalt iron oxide magnetic nano-particles (CoFe2O4 MNPs) in rabbits. CoFe2O4 MNPs were synthesized by the conventional micro emulsion technique in crystallite size range of 30 to 50 nm. The lattice constant (a) and cell volume were found to be 8.386 A and 589.75 A3, respectively, revealed by XRD. Subject animals were divided in three groups--low dose, high dose and control group without nanoparticles implantation for biocompatibility evaluation. CoFe2O4 was intraperitoneally implanted in rabbits: low dose (1mg CoFe2O4/Kg body weight) and high dose (10mg CoFe2O4/Kg body weight). Blood, serum and histological study of vital organs (liver, heart, kidney and spleen) were carried out in seven days of time protocol after sacrificing of animals. Results indicated that CoFe2O4 had drastically affected the blood chemistry in a dose-dependent manner as RDWa (P=0.01), Platelet (P<0.001) and Plateletcrit (P<0.001) concentrations reduced significantly in low dose and high dose CoFe2O4 treatments as compared to sham treated control group. Histological analysis revealed that CoFe2O4 exposure resulted in disordered and abnormal histology of liver, kidney and that of muscles at surgical site. It is concluded that CoFe2O4 has low biocompatibility and higher toxicity levels in living system at the applied doses.