L1(0)-orderd FeNi alloy films with a high degree of order (S) and a large uniaxial magnetic anisotropy energy (K-u) were realized by denitriding FeNiN films. FeNiN films with the a-axis perpendicular to the film plane were epitaxially grown on SrTiO3 (001) substrates by molecular beam epitaxy by changing the growth temperatures (T-S) to 200, 250, and 350 degrees C. The a-axis oriented epitaxial L1(0)-FeNi films were fabricated by annealing the FeNiN films in a H-2 gas atmosphere at 300 degrees C. S and K-u of the denitrided L1(0)-FeNi films were characterized by anomalous x-ray diffraction using synchrotron radiation and magnetic torque measurements, respectively. A high S of 0.87 and a K-u of 5.9x10(5) J/m(3) were realized in the L1(0)-FeNi film with a T-S of 350 degrees C. This high S value exceeds the values reported on L1(0)-FeNi to date, but the K-u value was comparable to those of c-axis oriented L1(0)-FeNi films with S similar to 0.5 grown by alternate monoatomic deposition of Fe and Ni layers. A possible origin for the suppressed macroscopic K-u in a-axis oriented L1(0)-FeNi films is discussed, and denitriding FeNiN is a promising method for the fabrication of L1(0)-FeNi with a high S and a large K-u.