Polycarbonate is one type of polymer known as strength and is easily shaped, and has a transparent color like glass. To increase the need for a more advanced form of processing in connecting technology, the Friction stir welding method was chosen. Friction stir welding is the process of joining two pieces of material by utilizing the tool's friction heat with the workpiece surface. This research was conducted to determine the effect of ratio of shoulder diameter and tool depth pin plunge to the mechanical properties of friction stir welding. This study uses polycarbonate material with dimensions 150 x 100 x 5 mm, the variation used in this study is the ratio of shoulder diameter 10/3 mm / mm, 14/3 mm / mm, 16/3 mm / mm with 4,6 mm plunge depth and 4,8 mm. Parameters that are kept constant are; tool rotation speed 2350 rpm, feed rate 14 mm / minute, and shape of screw cylinder pin tool. Tests conducted are; macro structure testing, hardness testing and tensile testing. The maximum tensile test results were obtained on the connection variations in the shoulder diameter ratio 14/3 mm / mm with a 4,6 mm plunge depth 31,54 MPa or 38,05% of the tensile strength of raw material. The maximum hardness value is obtained by varying the shoulder diameter ratio of 14/3 mm / mm with a 4,8 mm depth plunge of 80 Shore D. In testing the macro structure the variation in the ratio of shoulder diameter is 10/3 mm / mm with 4,6 mm depth plunge showing the surface the rough and uneven, there are lots of flash, the cavity defects and the welding grooves are not completely filled. Optimizing the ratio of shoulder diameter and pin immersion depth results in a uniform spread of melting material, so as to minimize the occurrence of defects and increase the mechanical strength of the joint.