Abstract: Microdefect characteristics in TC4 linear friction welding joints in relation to amplitude was studied from the standpoint of mechanics. The results indicated that the type of microdefect characteristics included micro pore which were formed in the partly fused joint interface and three-branch structure caused by the separation of the vibration direction flash and the vertical vibration direction flash. The coupled mechanical effects resulted from friction pressure and friction shear force dominated the formation of three-branch structure. The coupling of welding heat input and metal space shift governed the size of micro pore. Other welding parameters remained constant and there was a critical amplitude,A_cr,in all of experimental amplitudes A. On one hand,minimum branch length and maximum branch angle appeared and maximum micro pore diameter decreased continuously with the increase of amplitude when A≤A_cr. On the other hand,branch length and branch angle decreased continuously and maximum micro pore diameter remained constant with the increase of amplitude when A>A_cr.