Butt fusion welding is an effective method for joining plastic pipe on job sites. High density polyethylene (HDPE) pipes are widely used in many sectors such as water supply, natural gas distribution, petrochemical pipeline and service water system in nuclear power industry due to its excellent characteristics of low weight, high flexibility, high ductility and good corrosion resistance. This paper studies the effect of welding parameters on the tensile strength of the HDPE pipe butt-fused joints. The study was realized on HDPE pipe butt-fused joints that were made under different time, temperature and pressure at different stages of welding cycle. This study found the optimum welding parameters for HDPE pipes with the diameters of 75 and 110 mm. The optimum welding parameter permitted reducing the welding time.