Friction stir welding (FSW) is a welding process in a solid state that does not require additional materials. Its connection utilizes the friction of the rotating pin tool with the workpiece to make it connected. HDPE is a polymer material that can be connected using the FSW process. The purpose of this study was to determine the effect of tool rotational speed on the mechanical strength of HDPE materials by knowing the macro structure, tensile characteristic, and flexural characteristic. The parameters used are variations in the rotational speed of 900 rpm, 1500 rpm and 2000 rpm. The welding process uses a feed rate of 20 mm / minute, depth of plunge 0.5 mm, and pin tool of 3 mm. Tensile test using ASTM D638 type IV standard, flexural test with ASTM D790 standard and macro structure test. The higher the speed tool, the lower the mechanical properties of HDPE material. The highest tensile strength was obtained from a 900 rpm tool rotating speed of 13.6 MPa with a 6.2% strain and 0.51 GPa modulus of elasticity, at a 2000 rpm was 2.69 MPa with a 4.6% strain and a modulus of elasticity of 0.21 GPa. The highest flexural strength value of 14.2 MPa at 900 rpm, at 2000 rpm of 8.58 MPa. In observing the macro structure, every variation was seen with flash defects, but at 2000 rpm rotational speed was seen with many defects namely void defects, flash defects, and dirt in the weld joint area.