Abstract: Heat production mechanisms in temperature ranges below and above Tg(glass transition temperature) of thermoplastic components were studied by numerical simulation and experiment.The viscoelastic heat and facial friction heat in ultrasonic welding of PMMA(polymethylmethacrylate) were numerically calculated by FEM(Finite Element Method).Temperature was measured to verify the simulation results.Results of simulation and experiment agree well with each other,which in dicate that the facial friction heat is the initial heat source in ultrasonic welding process.Heat conduction effect chain-reactingly activates the generation of the viscoelastic heat when temperature reaches Tg of the base metal.And the viscoelastic heat provides most required heat during welding.The present study gives a more clear understanding of heat production mechanisms in ultrasonic welding.