We implanted 6 X 10(16)-3 X 10(17) nitrogen ions/cm(2) into 100 nn thick tungsten thin films with acceleration energies of 20-60 KeV. As a result, the thermal stability of N+-implanted W thin films is greatly improved from 700 to 900 degrees C because polycrystalline W thin films change into nanostructured films after N+ implantation. The W thin film implanted at 40 KeV and 3 X 10(17) ions/cm(2) effectively prevents Cu diffusion after an annealing at 800 degrees C for 30 min. When the acceleration energy and dosage are higher or lower than this optimum condition, thermal stability of the N+-implanted W film is degraded due to surface damage of Si substrate and partially nanostructured W thin film.