A method for evaluating a residual fatigue life of mechanical parts, consisting of the steps of grinding a surface layer of a mechanical part to be inspected by a minute amount to form an inspection surface, measuring half-width data of an X-ray diffraction intensity curve on the inspection surface, calculating a depth (do) of a fatigue damaged region from a graph of a half-width ratio (H/Ho) versus a depth (d) below the surface layer, and determining a fraction of fatigue life N/Nf on the basis of data of the depth (do) of the defective region versus the fraction of fatigue life N/Nf which were separately obtained from a test piece.