Abstract: Influence of presetting nickel base alloy sheet on microstructures and mechanical properties of 15CrMo/1Cr18Ni9Ti dissimilar steels joint by ultra-narrow gap welding（UNGW） was studied. The results showed that when the thickness of nickel base alloy sheet was less than0. 6 mm,part of weld zone near the fusion line solidified with A mode and formed austenitic cellular crystals,while the other zone still solidified with FA mode and was made of austenite equiaxed grains and dendrite ferrite. No solidified cracks appeared in the center of weld. When the thickness of nickel base alloy sheet was 0. 9-1. 2 mm,the center of weld zone solidified with A mode due to Ni macrosegregation,where coarse austenitic columnar crystal and solidification crack formed. When the thickness of presetting nickel base alloy sheet was 0. 6-0. 9 mm,Ni mass fraction of weld zone near the fusion line was 2% higher than that of filler metal（ER347L）,which brought better effect of inhibiting carbon diffusion than that of the joint without presetting nickel base alloy sheet. However,when the thickness of presetting nickel base alloy sheet changed in the range of 0. 3-1. 2 mm,the effect of joint on inhibiting carbon diffusion had no significant change. Dissimilar steel welded joints had martensite layer in the uneven mixed zone,while a certain width of carbon-rich hardening zone and carbon-poor softening zone formed near the fusion lin after heat aging treatment.