Abstract: Geometric information is important for automatic program ming of arc welding robot.Complete geometric models of robotic arc welding are established in this paper.In the geometric model of weld seam,an equation with seam length as its parameter is introduced to represent a weld seam of any shape.The method to determine discrete programming points on a weld seam is presented.In the geometric model of weld workpiece,three class primitives and CSG tree are used to describe weld workpieces.Detailed data structure is presented.In pose transformation of torch,world frame,torch frame and active frame are defined,and transformation between frames is presented.Based on these geometric models,an automatic prog ramming software package for robotic arc welding,RAWCAD,is developed.Experiments show the geometric models are practical and reliable.