Polycarbonate is a group of thermoplastic polymer which one easily formed using heat. This plastic has many advantages, such as thermal resistant higher than other types of plastic, resistant to impact, and is very clear. related to this case, the need for a form of processing on a technology will also increase which can be applied to the material, so the material is more efficient. One of them is the friction stir welding method. Because this method has a very high potential in the thermoplastic joining process. One of the thermoplastic is polycarbonate. Pin tools diameter value and rotational speed are a factor which affects the results of the joining process of friction stir welding. such as mechanical properties and material structure of joint results. this research was conducted to determine the effect of pin tools diameter and rotational speed on material structure and mechanical strength on friction stir welding using polycarbonate. The material used in this research is polycarbonate sheet with a thickness of 5 mm, length 150 mm and width 100 mm. Joining material is done with rotational speed of 985, 1660, 2350 rpm, feed rate of 16 mm/ minute, depth of plunge of 4,8 mm, the pin tool used is threaded cylinder with diameter value 3 and 4 mm. On this research 3 (three) tests were carried out including tensile testing, hardness testing, and macrostructure testing. The results showed that using pin tools with a diameter of 4 mm got the best results where the surface results of the material looked more evenly distributed and seen in the macrostructure only had a slight defect in welding. For the value of the largest tensile strength obtained 27,174 MPa and strain value 2,045%. Whereas for base material the value obtained for tensile strength is 82,87 MPa and strain 4,86% where the highest value of welding results is obtained 32,791% tensile strength and 42.078% strain from the base material. Based on the results obtained above, the recommended pin tool is a 4 mm diameter and 985 rpm engine speed because in this variation surface results and material mechanical properties are better than other variations.