Friction Stir Welding (FSW) is an alternative method of joints in the welding field. The FSW method is done by combining two clamped workpieces and then utilizing the heat caused by friction between the rotating tool and workpiece that forms the melt and mixture thoroughly so that a welded joint is formed. FSW can reduce solidification problems such as fusion welding, and also produce welded joints with functional properties, even in materials that do not fit with fusion welding. Not only metal, but this joints method also can be applied to polymer materials such as nylon 6. The purpose of this study was to determine the effect of tool diameter ratio and feedrate on the nylon 6 sheet with the FSW method on the tensile properties of the joints. In this study the specimens used was nylon 6 material with a dimension of 115 mm x 100 mm and thickness of 4 mm using ST80 based tool, then jointed through the FSW process with various parameters in the tool diameter ratio of 10/3 mm/mm, 15/3 mm/mm, 20/3 mm/mm and feedrate of 4 mm/minute, 6 mm/minute, 8 mm/minute. After jointed, then the weld joint is cut by using water jet cutting according to the tensile testing standard ASTM D638 type IV. After completing the specimen making process, then the samples were tested. This test included macro photographs testing, hardness testing, and tensile testing. The result of this study is the highest tensile test value obtained at the variation tool diameter ratio of 10/3 mm/mm with the feedrate of 6 mm/minute that is with tensile strength value of 19 MPa. From the observation result of macro photographs, it is known that the most defects are in the variation tool diameter ratio of 20/3 mm/mm defects that occur including incomplete fusion, flash, thining and lack of bonding. The result of the FSW joint hardness test on parameter variation with the highest tensile strength amounted to 54.5 HD. The results show that the parameter of tool diameter ratio and feedrate affect the tensile properties and hardness values.