The standard recommends for electrofusion welding of polyethylene pipes, used for natural gas and water transportation network to be carried out at the temperature from -5 degrees C up to +30 degrees C. In this study, an effect of the environment temperature on the properties of weldment is investigated. For this purpose, high density polyethylene (PE100) pipes were joined with electrofusion welding method at the various environment temperatures (room temperature, 0, -10, -20 and -30 degrees C) as received condition and by using preheat treatment (60 degrees C). According to standard, leaking and crushing test has been carried out and the double decohesion peeling test has been also applied for determining the strength of weldment. Thus, decohesion toughness, K, and nominal energy E-n, for decohesion of weldment calculated. Conclusively, the strength of the weldment increased meanwhile the ductility decreased by increasing crystalline amount in weld region associate to increasing environment temperature. It was determined that the pre-heat treatment process affected positively features of weldment which was joined in low environment temperature meanwhile; the strength of weldment joined at room temperature is affected negatively.