<p>Corrosion-induced reduction in clamping force threatens the integrity of friction-type bolted connections in steel bridges. This study proposes a non-destructive method for evaluating the condition of corroded high-strength bolts based on X-ray diffraction (XRD) analysis. Traditional techniques, such as ultrasonic testing or strain gauges, require precise bolt geometry or bolt removal, making them impractical for in-service structures. A finite element (FE) analysis was first conducted to examine how bolt-head thinning affects stress distribution. The results showed that reductions in head height, rather than width, significantly influence stress distribution and clamping force evaluation. To validate these findings, XRD measurements were performed on bolt-head surfaces with different levels of head height reduction. A strong correlation was observed between surface stress and clamping force, with the slope of this relationship changing according to the degree of thinning. Based on the experimental results, a calibration equation was developed to evaluate the clamping force while accounting for head thinning. The method demonstrated an evaluation accuracy within ± 40 kN, which is comparable to conventional methods used for new bolts, although variability remains due to initial residual stress. A major source of variability is considered to be the residual stress present from manufacturing. Despite this, the proposed method enables non-destructive evaluation of bolt condition and is primarily suitable for screening and maintenance planning of large-scale bridge structures.</p>