Abstract: A data structure is established to reconstruct the topology of triangular meshes in STL file. Issues concerning boundary extraction and facets segmentation are addressed in detail.A method for recognizing regular surfaces such as cylindrical and spherical surface is provided.Thus,the original shape of mechanical parts can be resumed.For freeform shapes,NURBS surface is reconstructed by means of harmonic parameterization,and an optimal method for ensuring G¹ continuity between adjacent patches is also provided.Practical examples demonstrate the feasibility of proposed approach.It is very useful for NC machining applications.