Abstract: The microstructures of 40Cr and T10A steel and its surfaces were ultra-fined through salt-bath cyclic quenching,high frequency hardening and laser hardening,then influence of the welded surface microstructure of steels on the process and joint quality of isothermal superplastic solid-state welding(ISSW) for 40Cr/T10A dissimilar steels was studied.The experimental results showed that the welded surface microstructure of steel plays an important role in the formation of ISSW joint.The welding temperature of ISSW extends to low temperature ranges and the initial strain rate extends to higher strain rate ranges while having finer welded surface microstructure of steel before welding.If only one surface is eligible for ultra-fining,the tensile strength of the joint will be also up to that of 40Cr base metal,but the welding time is slightly longer.ISSW belongs to the little deformation welding,the deformation of joint is mainly located in the quenched area near original interface.The deformation of T10A side is larger than that of 40Cr side.