In this paper photothermocapillary (PTC) effect has been used for the diagnostics of the dimensions of the foreign subsurface high-conductive inclusions in a low conductive substrate. A modeling system was an ebonite disc with built-in flush vertical and horizontal copper plugs with diameter 2,2 mm. Maximum length of the vertical plug is lpl = 14 mm and horizontal – lpl = 52 mm. The working surface of the substrate was covered with a film of the dried black zapon lacquer absorbing the radiation of the pump laser beam (maximum Pe = 10,4 mW). A thin layer of silicone oil PMS-5 (h = 300 μm) was deposited on the working surface of the ebonite disc to generate of the PTC effect. Using a low-power probing unfocused laser beam we measured the diameter Dst of the stationary PTC signal in dependence on lpl. It was found that for the vertical plugs the method has a high sensitivity ΔDst/Δlpl = – 120 for lpl ≤ 1,6 mm, and for horizontal - maximum sensitivity ΔDst/Δlpl = – 79 is achieved for lpl ≤ 3 mm. Further increase in lpl does not lead to a noticeable change in Dst that is obviously associated with reaching a maximum of lpl that corresponds to a thermally semiinfinite plug. For case of the horizontal plugs an increase in Pe from 3,9 to 10,4 mW increases the slope ΔDst/Δx at the boundary in between ebonite and plugs for 9 times.