Abstract: A relationship between internal friction(IF) and microstructures of high silicon martensitic hot working die steel(SDH3 steel) which contains 1.5% silicon was researched.The experimental steel was annealed at 1060℃ for 30min,quenched into oil and tempered at different temperatures.The temperature dependence of internal friction(TDIF) was measured with a vibrating-reed apparatus by free decay method within the temperature range from room temperature to 750℃.It is shown that two types of relaxation IF peaks,Snoek peak and SKK peak,have been observed at TDIF curves of the steel.The height of IF peaks drops and the peak position shifts as the tempering temperature rises.When the tempering temperature is above 650℃,the IF peaks disappear completely and only background IF can be observed.As the tempering time is prolonged,both the peak height and peak position change,and then these changes have been discussed on the basis of diffusion-controlled redistribution.