Abstract: Transverse cracks were found on circumferential weld of tubesheet and shell ring of an unused high pressure heater. Causes of cracks were analyzed by macroscopic inspection, chemical composition analysis, hardness testing and metallographic examination. The results show that the cracks located in the fusion zone of tubesheet, generated in the coarse grain area of heat affected zone (HAZ), they extended along grain boundaries and were perpendicular to the circumferential weld. The cracks were welding cold cracks with typical characteristics of hydrogen induced crack, and large internal stress produced by cooling shrinkage of weld seam also promoted the initiation and propagation of cracks.