Carbon deposition on specimens exposed to electron beams in an electron-microscope chamber, which is offer used to measure the size of ULSI patterns on silicon wafers, is very serious problem. We measured the increase of aluminum wiring-line width on wafers due to carbon deposition by directly spreading oils on the wafers. This method indicated that, linewidth increase varied from 1 to 200 nm. This method is a useful way to estimate carbon deposition since it is highly sensitive and does not get contaminated.