Inconel 718 with thickness ranged from 0.1-1.7mm was chosen as interlayer to promote weldability in friction welding of TiAl intermetallics and structural steel such as AISI 4140, in which the welded joint presents single fin showing less welding deformation on TiAl side. The correlations between tensile strength and the interlayer thickness were analyzed and fitted to a model. It indicates an optimum interlayer thickness ranged from 0.9-1.1mm where the tensile strength reaches as high as 360MPa. Otherwise, while the interlayer thickness decreases to 0.1mm, brittle compounds of TiC, Al_2Ti_4C_2 and M_7C_3 are formed in the welded zone so that the tensile strength decays. Thicker interlayer should be also avoided as double joints may occur at TiAl-Inconel 718 and Inconel 718-AISI 4140, respectively, which lowers the tensile strength to some extent. Inconel 718 with thickness ranged from 0.1-1.7mm was chosen as interlayer to promote weldability in friction welding of TiAl intermetallics and structural steel such as AISI 4140, where which welded joint presents single welding less on TiAl side. The correlations between tensile strength and the interlayer thickness were analyzed and fitted to a model. It indicates an optimum interlayer thickness ranged from 0.9-1.1mm where the tensile strength reaches as high as 360MPa. mm, brittle compounds of TiC, Al_2Ti_4C_2 and M_7C_3 are formed in the welded zone so that the tensile strength decays. Thicker interlayer should be also avoided as double joints may occur at TiAl-Inconel 718 and Inconel 718-AISI 4140, respectively, which lowers the tensile strength to some extent