Abstract: Inertia friction welding process of 45 steel drill pipe was investigated and the effect of heat treatment process on the microstructures and mechanical properties was studied. The results show that the defect-free welded joints with good bonding quality were obtained at the friction time of 18 s,friction pressure of 3 MPa,and upsetting pressure of 7 MPa. The microstructures of welded joint are priority proeutectoid ferrite and pearlite,and that of the base metal are granular ferrite along grain boundary and pearlite,the fracture morphologies is brittle fracture mode with quasi cleavage. After heat treatment,the microstructures of the welded joint and the base metal are fine,the comprehensive mechanical properties are improved,and there are some dimples in the fracture appearance.