Abstract: Three dimensional observations of internal defects and inner surface quality for small diameter pipe welding seam were performed using X-ray micro-computed tomography. Five nearly spherical gas pore defects were found. The spatial position distribution of gas pores are demonstrated by three dimensional(3D) visualization, and quantitative analysis of three-dimensional characteristic parameters of gas pore defects, such as position, volume, diameter, sphericity, and so on. The results show that three gas pores are excessive defects, the remaining two gas pores are non excessive defects. A technique of three dimensional sectioning was introduced, and a surface pit with non excessive size was found in inner surface. Based on the 3D reconstruction model, the uniformity of welding seam thickness distribution was characterized by the cloud chart of wall thickness analysis. Research results shows that positioning, quantitative and qualitative analysis of the internal defects in small diameter pipe welding seam with high precision can be realized by CT.