The aim of this study is to investigate the effect of important process parameters and tool parameters on joint strength in fiction stir welding process of high density polyethylene (HDPE) using the Taguchi experimental design method. The tool rotation, feed rate and tool shoulder diameter were considered as varying parameter and used to fabricate the joints. The quality of the joint was evaluated by examining the characteristics of the joint as a result of ultimate tensile strength. For friction stir welding of the plates, the experiments were arranged by using Taguchi's L-9 orthogonal array in randomized way. From the analysis of variance and the signal-to-noise response graphs, the significant parameters and the optimal combination level of the parameters were obtained. It was found that using a tool rotation of 1600 rpm, a feed rate of 20 mm.mm(-1) and a shoulder diameter of 20 mm, an improved joint strength can be obtained. A confirmation run is also performed to prove the effectiveness of the Taguchi method. The results show that joint strength was improved by about 25 % compared with the initial welding parameters.