In the present work, friction stir welding technique was applied on 2 mm thick ferrite-martensite DP700 steel sheets at rotational speeds of 600, 800 and 1000 rpm. The microstructure and mechanical properties of the welds were evaluated. It was found that Zener-Hollomon parameter decreased with increasing rotational speed that leads to grain coarsening in the stir zone. It was also found that increment of rotational speed increased softening phenomenon in sub-critical heat affected zone. The results also showed that the presence of WC particles in the stir zone, which was due to the tool wear, as well as formation of a soft ferrite band degrade the tensile properties at rotational speed of 600 rpm while HAZ softening was responsible for reduction of strength and ductility in conditions of 800 and 1000 rpm.@en