Abstract: Severe porosity was found in 718H steel and 40Cr13 steel produced by some company. The forming reasons of center porosity of 718H steel and 40Cr13 steel were analyzed through methods of chemical composition analysis, macroscopic examination, inclusion examination, metallographic examination, etc. The results show that:the main reason for porosity of 718H steel was agglomeration of inclusions; the main reasons for porosity of 40Cr13 steel were precipitation and agglomeration of segregation solute elements; finally specific improvement measures were also put forward according to the porosity reasons.