Compared with previous work, FePt/B nanocomposite films with larger B content in a wide range were prepared by magnetron sputtering plus subsequent annealing on Si (1 0 0) substrates. When the volume percentage is no less than 12%, both interstitial incorporation and intergranular segregation of the B are found, resulting an enlargement of the FePt lattice and a decrease of the FePt grain size. Effect of B content on magnetic properties, such as magnetization, switching field and exchange coupling of the FePt based films, is also discussed.