The particle size and magnetic properties of Co-B nanoparticles in polymeric resin with B concentrations of 3.04 and 4.38wt% before and after heat treatment were investigated by various techniques. X-ray diffraction results reveal that the Co-B nanoparticles are a mixture of part crystalline and part amorphous phase. Transmission electron microscopy shows that the particle size of Co-B nanoparticles encapsulated in resin is increased by the increase of Co 2 + ions exchange and reduction times. Decomposition temperature of the resin is about 460 o C, so the heat treatment effect at 250 and 390 o C on their magnetic properties is also studied. X-ray diffraction and vibrating sample magnetometer results demonstrated that the crystallinity, magnetization and coercivity can be affected by heat treatment temperature.