X-Ray fractography is a useful technique to analyse the mechanisms operating in fracture and involves an examination of the fracture surface. In the present investigation, this technique has been employed to study the fatigue fracture behaviour of a medium carbon steel of C45 grade in different heat treated conditions. The different trends observed in the residual stress (σr) and diffraction profile full width at half maximum intensity (B) relationships with the maximum stress intensity factor (Kmax) on the fracture surface have been correlated to the differences in flow characteristics of these materials. The root mean square value of microstrain , and the coherent domain (particle) size, D, were determined through single line diffraction profile (Voigt's) analysis. It has been observed that contribution of microstrain to profile broadening is more significant than that due to domain size. However, at higher Kmax values an influence of D was found on line broadening, to a small extent. Results of sub-surface stress measurements were compared in two conditions. A good correlation was noticed between the depths below the fracture surface, designated as ymax, at which the measured σr, reaches the base material value and the corresponding monotonic plastic zone size (rp) obtained by calculation. The observed depths below the fracture surface (ypeak) at which an increase in σr or a decrease in B was noticed, appear to be related to the conditions of the near-tip regions where the material has undergone severe fatigue damage and cyclic softening.