This paper presents the effect of fusion procedure on the fusion performance of the thermal butt fusionin the safety class III buried HDPE piping per various tests performed, including high speed tensileimpact, free bend, blunt notched tensile, notched creep, and PENT tests. The suitability of fusion jointsand qualification procedures was evaluated by comparing test results from the base material and buttfusionjoints. From the notched tensile test result, it was found that the fused joints have much lowertoughness than the base material. It was also identified that the notched tensile test is more desirablethan the high speed tensile impact and free bend tests presented in the ASME Code Case N-755-3 as afusion qualification test method. In addition, with regard to the single low-pressure fusion joint performances,the procedure given by the ISO 21307 was determined to be better that the one specified inthe Code Case N-755-3.