Abstract: Five pots of low alloy ship hull Mn-steel with and without rare earth treatment were made in laboratory,and a commercial manufactured Mn-steel with RE treatment was collected.The influence of the modification of inclusion on the pit initiation and pit propagation of the steel was studied by means of metallurgy analysis,polarization test,occluded corrosion cell(OCC)test and coupon test.The results suggested that the inclusions in the steel are main sites for pit initiation,and the resistance against pitting corrosion of steels is strongly affected by the magnitude of the sulfide inclusions.Less and fewer RE-sulfide multi-phase inclusion heavily decreases pitting susceptibility and rate of pit propagation,which improves the resistance of the steel to corrosion.