High density polyethylene (HDPE) is one of the popular polymers because of its availability and competitive costs. Along with this the need for more advanced forms of processing technology has also increased which makes material applications more convincing, one of which is joint technology. Friction stir welding (FSW) joint method is one of the joint methods that has great potential in joint thermoplastic polymers, especially HDPE. The shape of the pin tool is one of the important factors that influence the weld results in the FSW process, such as the structure and mechanical properties of the joint results. This research was conducted to determine the effect of pin tool shape on the mechanical strength of friction stir welding using high density polyethylene (HDPE) workpieces. This study uses HDPE sheets with dimensions of 5mm thickness, 80mm width and 100mm length. Welding is carried out with a rotation speed of 1560 rpm, a welding speed of 14 mm/minute, a dept of plunge of about 0.5 mm and a variety of pin tool shapes which are divided into two groups, namely 3mm and 4mm diameters, both of which have three forms of pin tools namely cylinders, threaded cylinders and triangle. Three tests were carried out in the study, namely the testing of macro structure, tensile strength and hardness of the weld results. The results of this study indicate that the use of pin tool shape 3mm threaded cylinders shows good results where the weld structure of the macro looks no defect and produces a flat surface and constant results in all areas of the welding process. The hardness value is obtained with a value of 41.47 HRH while the basic material is 54.77 HRH, thus it can be interpreted that the highest hardness value of the joint is around 75% of the base material. In the tensile strength test the maximum stress value reached 17.3 MPa with a strain value of 5.8%. While the stress on the base material is 25.9 MPa and strain is 13%. Thus the shape of the pin tool that is recommended based on the results of this study is the shape of the pin tool screw cylinder with a diameter of 3 mm, because it has good surface results and mechanical properties.