Abstract: Inertia friction welding technology was mainly used to connect medium carbon quenched and tempered steel 42CrMo with nickel base superalloy GH4169. Microstructure and mechanical properties of welded joints under different inertia were analyzed by means of optical electron microscope, scanning electron microscope, energy spectrum analyzer, microhardness tester and electronic tensile tester. The research results showed that when inertia was 168 kg·m², flash of 42CrMo and GH4169 inertial friction welded joints was well formed, and there was no obvious defect after flash was removed. Dynamic recrystallization occurred on both sides of weld interface, and uneven intermetallic compounds (Ni-Fe-Cr, Ni₃Al) and carbide layers (NbC, TiC) with thickness of 5 μm were observed, which also resulted in microhardness peak of 691 HV1.0 at the weld interface. According to tensile property test of welded joints, fracture location of samples under 168 kg·m² at room temperature and high temperature tensile (540 ℃) occurred on the 42CrMo base metal side. Fracture morphology observation showed typical ductile fracture, tensile strength at room temperature reached 1062.5 MPa, and tensile strength at high temperature reached 687.5 MPa, which were similar to 42CrMo base metal. This indicated that intermetallic compounds and carbide layers formed at the interface promoted metallurgical bonding of interface materials, and effectively improved mechanical properties of welded joints.