The effect of surface finish on the evaluated content of retained austenite on quenched AISI 52100 steel (Rc = 62) by X-ray diffraction (XRD) was investigated. The sample surfaces were prepared with different surface finishes under different load and vibratory polishing with colloidal silica suspension prior to XRD testing. The surface morphology of samples was observed by scanning electron microscope, and surface roughness was evaluated. The measured content of austenite increased with the surface roughness resulting from grinding surfaces with different surface finishes. The coarse surface grinding can deform the surface and transform some of the retained austenite into martensite. The deformation layer became thinner with finer grit. The thickness of the deformation layer can be as high as 20 µm due to coarse grinding and can be less than 7 µm due to fine polishing. The deformation layer would have a huge influence on the measurement of retained austenite due to low analyzed depth of XRD. In order to accurately evaluate the content of the retained austenite in the sample, minimizing the depth of the deformation layer should be ensured, either by fine polishing or electropolishing.