As　an　environmental-friendly　material　with　high　rupture　strain　capacity,　polyethylene　terephthalate　(PET)　fiber　reinforced　polymer　(FRP)　has　promising　industry　applications.　Previous　studies　mainly　focused　on　the　application　of　PET　FRP-confined　concrete　in　circular　columns,　while　investigations　on　PET　FRP-confined　concrete　in　square　columns　are　rather　limited.　The　purpose　of　this　study　is　to　fundamentally　understand　the　confinement　mechanism　of　PET　FRP-confinede　concrete　in　square　stub　columns　by　conducting　axial　compression　tests.　Particularly,　the　influences　of　confinement　thickness,　corner　radius　and　specimen　size　were　investigated.　Test　results　reveal　that　the　specimen　size　is　not　significant　for　small-and　medium-sized　stub　columns　with　a　similar　confinement　level.　The　minimum　confinement　stiffness　ratio　and　modified　confinement　ratio　for　achieving　a　sufficient　confinement　are　0.0036　and　0.13　respectively.　Additionally,　assessments　of　existing　confinement　models　for　PET　FRP-confined　concrete　are　conducted.