Abstract: This paper focuses on the causes of defects in the polyethylene oil tanks made by an imported polyethylene (PE) material through rotational molding. The transverse and longitudinal sections of the tanks and their internal and external morphologies and diameter distribution of pores were analyzed by SEM, stereoscopic microscopy and micro CT instrument. The results indicated that there were a large number of pores inside the tanks but few of them on the external surface contacting the mold. Moreover, there were also some pores found on the other sides, and most of the pores were distributed in the sub-outer layer, where the penetrating holes were found with a depth more than 1.9 mm and an overall porosity of 2.4 %. The gel content in each layer of the transverse section was higher than 60 wt%. A comprehensive investigation showed that the presence of abundant pores inside the defective tanks was due to the premature cross-linking of PE during the rotational molding process, which resulted in an increase of viscosity of the system. As a result, the bubbles were not expelled out of the tanks, thus leading to the porous defect. Some suggestions for the prevention of porous defect were finally proposed in this work.