High-density　polyethylene　(HDPE)　pipes　are　the　preferred　pipes　of　water　systems　in　nuclear　power　plants　because　they　are　durable,　corrosion-free,　easy　to　install,　and　not　subject　to　fouling.　However,　their　long-term　performance　can　be　affected　by　welding　defects.　In　this　paper,　the　effect　of　welding　defects　on　the　long-term　performance　of　HDPE　pipe　butt　fusion　joints　was　studied　using　a　creep　test.　A　welding　defect　with　a　hole　or　inclusion　in　the　joint　was　simulated　by　artificially　inserting　a　copper　ball　during　butt　fusion　welding.　The　test　results　showed　that　the　creep　life　of　the　joint　decreased　with　increased　defect　size.　An　expression　describing　the　creep　life　and　the　defect　ratio　was　obtained　according　to　the　test　results.　In　addition,　the　test　results　showed　that　the　creep　life　of　the　joint　without　a　welding　bead　was　about　50%　of　that　in　a　joint　with　a　welding　bead.