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Annealing behavior, at different annealing temperatures, of an ultrathin Mo layer located between a Ti film and Si substrate or deposited on the top of surface of a Ti film was investigated by Rutherford backscattering spectrometry (RBS),cross-sectional transmission electron microscopy (TEM) and energy dispersive X-ray spectrometry (EDS). In a Ti/Mo/Si structure, partially reacted film with layer structure of Ti-rich silicide/TiSi2/(Mo, Ti)Si2 on a Si substrate was formed after 550℃annealing for 30 min. The ratio of Mo to Ti in (Mo, Ti)Si2 layer decreases from near Si substrate upwards and becomes zero at about 20 nm away. In a Mo/Ti/Si structure,the surface Mo layer enhances the Si diffusion from the substrate during annealing.Mo bearing Ti rich silicide exists on the surface until 600℃ and then converts to (Mo,Ti)Si2 after 650℃ annealing, and the atomic ratio of Mo to Ti decreases from the top surface into Ti silicide film, and becomes zero at about 30 nm away from the surface.In both cases of interface Mo and surface Mo layer, the atomic ratio of Mo to Ti in the region of (Mo, Ti)Si2 was found to be very low, with an average value of less than 0.2. Low content of Mo in Mo containing teary silicide leads easily to the formation of the stable phase of C54 (Mo, Ti)Si2, which acts as a template for the formation of C54 TiSi2 beneath when Mo is deposited on the surface.