In manufacturing lines, quantitative inspection of residual oil on steel surface is really needed. We carried out experiments to detect the residual oil-film on the surface after quasi-degreasing-process by using Laser Induced Fluorescence (LIF). As a result, the two-dimensional distribution of the oil-film with thickness of nanometer scale could be measured in the wide range of area. It is clear that the residual oil-film-area measured by LIF can assess the quality of degreasing process quantitatively. The method which has higher measuring sensitivity and accuracy is superior to the conventional water stained test.