Abstract: The initial rotation speed of flywheel determines the welding heat input in inertia radial friction welding process, which has a decisive influence on joint shape. In this paper, inertia radial friction welding experiments of 37CrMnMo steel pipes were carried out at different initial rotation speed by taking 45 steel as radial ring. The effect of initial rotation speed of flywheel on joint shape was analyzed, and the proper initial rotation speed of flywheel to obtain the good joint shape was obtained in theory. The experimental results showed that the joint is good at the initial rotation speed of 520 and 560 r/min, and the fin with arc shape formed by means of continuously and uniformly squeezing the plastic metal from the friction interface in welding process. Based on the characteristics of radial friction welding temperature field, the proper initial rotation speed of flywheel to obtain the good joint shape was induced by estimating the laminated energy in the ring. The calculated result is n₀=548 r/min, which is in good agreement with the experimental results