Scanning thermal microscopy (SThM) was used to map thermal conductivity images to investigate the thermal property variation in the ultrafine-grained surface layer of a pure iron sample produced by ultrasonic shot peening. A theoretical approach, based on this investigation, was used to calculate the heat flow going into the sample from the probe tip, and hence, to estimate the thermal conductivities at different scanning locations. Experimental results and theoretical calculation demonstrate that the thermal conductivity of the ultrafine-grained surface layer is significantly affected by its microstructural features, and the cross-sectional profile of the thermal conductivity contrast could be used to deduce the thickness of the deformed surface layer produced by ultrasonic shot peening. (c) 2005 Elsevier B.V. All rights reserved.