Abstract: The effect of burn-off rate on the microstructure and mechanical properties in friction welded joint of low alloy constructional steel 35CrMo has been studied in this paper. The results show that the increase in burn-off rate and deceleration time of friction welding has improved the toughness of the weld.The mechanism of improving the toughness of weld is the finer grain size and the lower hardness of the weld.Also the microstructure of the joint has been analysed too.The fine structure of granular structure in the area near the weld has been studied by TEM.It has been indicated that the strengthening mechanism of the weld metal is the dislocation,a number of second island phases and fine grain size.By means of measuring the carbon content of the area near the weld, the effect of burn-off rate on the highest heating temperature of friction welding has also been investigated.