Stacking faults ͑SFs͒ in Co-based alloy grains in a Co 100−x M x ͑M: Cr, Mo, and W͒ film are evaluated by means of in-plane x-ray diffraction. Moreover, the correlation between SFs and uniaxial magnetocrystalline anisotropy K u is discussed in connection with the spin-orbit interaction. The ratio of the integrated intensities of the ͑10.0͒ to ͑11.0͒ diffractions corrected by Lorentz and atomic scattering factors has been proposed as an index for SFs in hcp films with a c-plane sheet texture. This ratio is equal to 0.25 for perfect hcp stacking, while it is 0 for perfect fcc specific stacking. It has a one-to-one correspondence with the probability of -A-B-C-atomic-layer stacking P fcc . Using this index, pure sputtered Co films are found to have a P fcc of 10%. The addition of only 5 at. % of Mo or W into the Co grains reduces P fcc to 2%. K u was found to increase with the addition of material ͑e.g., K u was 4.0ϫ 10 6 ergs/ cm 3 for 5 at. % Mo͒, although the atomic magnetic moment of Co decreases monotonously. A P fcc of 10% is found to lower K u in a pure Co film by more than a factor of 2 when the spin-orbit interaction is taken into account.