The aim of this article is study of tool plunge depth (TPD) effects on mechanical properties of friction stir welding of AA1100 aluminium alloy to A441 AISI steel. For this purpose the 0.1, 0.2, 0.4, and 0.6 mm TPD selected and other welding parameters kept constant. The results shows that the frictional heat increases and stir zone grain size decreases with increasing TPD at both base metals. At higher TPD, the material press out from shoulder and base metals interface. The highest tensile strength allocated to the joint which welded whit 0.2 mm plunge depth. This joint had appropriate joint efficacy, material flow and microhardness compare other joints.