Abstract: The welding liquation cracks and the change of grain boundaries in HAZ of the cast Ni-base superalloy have been investigated by wide variety examinations,including metallograph,phase analysis,SEM,TEM,EPMA,etc.The mechanism of liquation cracking in some cast Ni-base superalloys is as follows.Under high welding temperature,the γ+γ' eutectic nearby the grain boundaries and the Zr rich low melting constituents in the front of eutectic reaction zone melted,and TiC nearby grain boundaries dissolved or partially dissolved and formed the constitutional liquation;these liquid phases flowed along the grain boundaries and the liquified grain boundaries,thus enhanced cracking under contracting stress.According to the above mentioned mechanism of liquation cracking in cast Nibase superalloy,the composition of M38 cast Ni-base superalloy was modified and its susceptibility to liquation cracking has been improved,some cther preventive measures against cracks were adopted and good results were obtained.