Abstract: The crack morphology of X90 pipeline steel weld, made by tack and finish welding, was investigated by mechanical test, microstructure analysis, SEM and EDS. Furthermore, the causes of the crack were analyzed. The results show that the transversal cracks in the inner weld bead of X90 helical welded pipe are cold crack, that initiated and grew transgranularly or along the grain boundary in the weld. The X90 steel contains a lot alloying elements such as Mn, Cr, Mo, Ni and Cu, and its strength is high. So the welding residual stress is larger than the crack critical stress. The Vickers hardness in the inner weld bead is higher than that in the outer weld bead, and the distribution of hardness on two sides of weld is very asymmetry. These results led to stress concentration and unbalance of distribution in the welded joints. Diffusible hydrogen, which highly existed in the overlap regions of inner and outer weld bead, escaped hardly from the weld metal. Therefore, the diffusible hydrogen gathered in the trap of weld defects such as gas hole and slag. This resulted in forming of cracks and lots of white splotches on the fracture surface. The continuous and elongated dendrite grain boundary, which was formed in the first crystallized microstructure of weld metal, provided a channel for the growth of crack.