Nanoparticles are widely applied as diagnostic reagents, drug-delivery carriers or bioaffinity beads. In particular, magnetic particles have been widely used in the study of biomedical applications. When we look ahead to the future, it is needed to construct precisely designed surface containing such as high bioaffinity and capability to capture of small amount protein or specific protein among various protein. As a method to construct precisely designed biocompatible polymer surface, atom transfer radical polymerization (ATRP) was widely used as a living free radical polymerization. ATRP has advantages such as control of molecular weight, narrow polydispersivity, easy synthesis of block copolymer, and preparation of high density polymer brush under mild condition. Previously, we have demonstrated synthesis of poly (2-methacryloyloxyethyl phosphorylcholine (MPC)) brush on ferric oxide (Fe3O4) by ATRP from immobilized initiator includes bromine that ATRP moiety. Here, it is known that poly(MPC) has been not affected for reduction of nonspecific binding, salt concentrations, pH and temperature. In this state, the poly(MPC)-grafted Fe3O4 could not be used for diagnostic material and collection material using magnetic interaction because poly(MPC) suppress protein adsorption. To introduce immobilization part for protein, p-nitrophenyloxycarbonyl polyethylene glycol methacrylate (MEONP) monomer that has active ester was polymerized as a block component. The poly(MPC-b-MEONP)-grafted Fe3O4 could capture proteins via active ester to amine of proteins. However, on careful consideration, this immobilization method had disadvantage for capture protein because immobilization part located in the side chain, not locate most outer surface. We focused on bromine (Br) located on end group of poly(MPC) brush, probably locate most outer surface. If the end group would be modified easily, the technique will be most effective modification. We adapted the reduction from azide (N3) to amine (NH2), after Br was exchanged to N3. This reduction has been called “staudinger reduction”. Generally, Click chemistry that reaction between N3 and alkyne is famous as a reaction of N3. On the other hand, the modification technique has wide application range due to amine group. At first, bromine was converted into azide, N3 using NaN3. Furthermore, the N3 was reduced to NH2 using PPh3 and H2O. The purification of magnetic nanoparticles with end group modified poly(MPC) brush was easy because of use of external magnetic interaction. The surface characterization at each step was estimated by XPS and FT-IR spectrum measurement. The absorbance signal attributed to N3 was appeared around 2000 cm-1 from FT-IR spectrum and N1s signal according to NH2 was observed at 399 eV from XPS measurement. The surface NH2 could react to carboxyl group or convert into other functional groups easily. The end group modification will be a useful for various bioaffinity bead applications.