The　transport　properties　in　ultrasmall　single-wall　carbon　nanotubes　(SWCNTs)　under　tensile　strain　have　been　theoretically　investigated.　The　regular　negative　differential　resistance　(NDR)　induced　by　the　strain　undergoes　a　process　from　enhancement　to　weakening　in　the　zigzag　(3,　0)　SWCNT.　The　NDR　achieves　maximum　with　applying　4%　tensile　strain.　Compared　to　the　case　of　(3,0)　SWCNT,　that　NDR　cannot　be　manipulated　by　applying　strain　clearly　in　(4,0)　and　(5,0)　ultrasmall　SWCNTs　with　tensile　strain　lower　than　10%.　It　proposes　this　strain-induced　NDR　effect　to　demonstrate　the　possibility　of　finding　potential　applications　in　SWCNT-based　NDR　nanodevices　such　as　in　memory　devices,　oscillators　and　fast　switching　devices.