Abstract: The microstructure of linear friction welded TC11-to-itself and TC11/TC17 dissimilar titanium alloy joints were investigated using scanning electron microscopy(SEM) and transmission electron microscope(TEM) to understand the formation mechanism of the resultant joint. The results show that the formation process of linear friction welded joint for titanium alloy to itself was different from that for dissimilar titanium alloy, because of the different behavior of heat generation. The heat was generated by friction at the beginning stage of linear friction welding of titanium alloy to itself. After forming metallic bond at the rubbing interface, the heat generation was controlled by material deformation. For linear friction welding of dissimilar titanium alloys, the heat was also generated by friction at the beginning stage, but the heat generation was mainly controlled by the deformation of TC11 alloy after forming metallic bond at the rubbing interface. Subsequently, the heat was conducted into TC17 alloy and led to its deformation. As a result, the heat generation was mostly related to the deformation of TC11 and TC17 alloys.