Abstract: The susceptibility to SR cracking of pressure vessel steel with Mn-Mo-Nb has been evaluated by the use of Y-type groove SR crack-resisting test and other testing methods.The influence of trace boron on the susceptibility of this steel to SRcracking has been studied by means of Scanning Electron Microscope,X-Ray Structure Analysis and a-particle traces autoradiography.It was found that trace boron increases obviously the susceptibility of Mn-Mo-Nb steel to SR cracking,depending mainly on the preeipitation and coalescene of carbon-boride on the grain-boundary of the coarse-grained HAZ,and consequently also on the decrease of high temperature creep ductility of the coarse-grained zone.This has been confirmed by the typical morphology of SR cracking of steel with boron.This paper,based on the analysis of the formation-mechanism of SR cracking,put forward the ways and means of controlling the "boron phase" and preventing SR cracking.