his paper is the first to optimize the friction stir welding process of pipes (FSWoP) considering the tool pin diameter effect using the multi criteria decision making (MCDM) techniques. Significant parameters of the FSWoP process have been considered in many optimization studies, however, the effect of the tool pin diameter has been never studied. Experimental work was performed using a new modified fixture to eliminate the post-process problems. Experiments were performed at three levels of three parameters: tool pin diameter tool, rotational speed and travel speed. For optimization, the MCDM methods were applied as an approach for selecting the optimal values of the parameters. Using MCDM in optimizing the FSWoP process is rare and has a limited number of publications. The MCDM techniques used are the Grey Relational Analysis (GRA) and Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS) to find the optimum process parameters which maximize the values of responses: the tensile strength and the hardness of the welded joint. The same results were obtained through both techniques and the optimum values for the used parameter ranges were 5 mm, 1800 RPM and 10 mm/min for the pin diameter, rotational speed and travel speed respectively.