Abstract: A method of 3-D defection for surface defects is presented with a light beam emitted by a linear laser, which is projected on the surface of steel plates vertically, and the surface images of steel plates are captured by an area-scan camera. Midpoint thinning is applied, and laser lines are extracted according to stabilization points. Partitioning calibration is proposed for calibration of the camera, and space coordinates of the laser lines are calculated based on the concept of space revert. Referential planes for depth calculation of defects are selected by projection scanning. The method was examined with defect samples, and the result of depth calculation was very close to experimental value.