Abstract: In this paper,a new method of labeling line drawing is proposed based on relations between L-junctions and W、Y-junctions. The labeling process in line drawings is from covered lines to uncovered lines. Some rules for judging covered lines are established based on the projective theory. Our labeling algorithm can discriminate between correct and incorrect hidden-part-draw natural line drawings by judging whether labeling of trihedral vertices is reasonable junctions or not. The effectiveness of the algorithm is demonstrated by the experiment. This algorithm makes computers easily understand and interpret line drawings.