Abstract: Failure analysis of welding cracking on ethylene cracking tubes by electron beam welding was carried out by means of metallographic analysis and SEM observation, etc. The results show that the reasons for the cracks formation were those the columnar crystals crossed line perpendicular to the weld center were formed during the electron beam welding and a large number of carbides precipitated from the weld joint centerline for the poor Nb, which led to the obvious brittleness.The high cooling speed after boiler shutdown led to great stress, the crack formed on the weld centerline and then grew and cracked.