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The anisotropic magnetoresistance (AMR) effect was systematically investigated in epitaxially grown Co 2 Fe x Mn 1−x Si films against Fe composition x and the annealing temperature. A change of sign in the AMR ratio from negative to positive was clearly detected when x increased from 0.6 to 0.8. This sign reversal can reasonably be explained by the change in the dominant s-d scattering process from s↑ → d↑ to s↑ → d↓ caused by the creation of large d-states at the Fermi level, suggesting the disappearance of half-metallicity at x = 0.8. The variations in the remanent density of states in the half-metallic gap against annealing temperature are also discussed from the viewpoint of the AMR ratio on the basis of the s-d scattering model.