High-density polyethylene (HDPE) is increasingly recognized as an eco-friendly material for small crafts. However, there is limited research on welding distortion in HDPE welded structures. This study investigates the deformation behavior of HDPE fillet-welded specimens, focusing on global out-of-plane distortions such as angular and bowing distortions. Experimental analysis, conducted using an optical 3D scanner, reveals similarities between the observed deformation patterns in HDPE fillet-welded specimens and those commonly found in other fillet- welded structures. Angular and bowing distortion modes are primarily observed, resulting from the contraction forces that occur during the cooling process after the melting and solidification of the HDPE welding rod. Additionally, the influence of specimen thickness on deformations is examined. This research contributes valuable knowledge about welding distortions in HDPE, providing directions for optimization of welding processes and the enhancement of structural performance in HDPE welded structures.