Using　the　addition　of　an　O2　to　a　(10,　0)　tube　as　a　model　case,　the　link　between　pressure　and　the　reactivity　of　a　carbon　nanotube　is　examined　by　first　principles　calculations.　Attaching　functional　groups　to　nanotube　side-walls　have　been　envisioned　as　a　method　to　produce　desired　structural　and　electronic　properties,　although　challenges　remain　for　activating　and　controlling　such　reactions,　especially　for　tubes　with　large　diameters.　Upon　pressure,　the　circular　section　of　a　nanotube　is　flattened　and　differentiation　in　chemical　reactivity　is　induced,　with　the　bent　section　more　reactive　than　the　flat　section.　In　the　reaction　with　singlet　O2,　both　the　adsorption　energy　and　the　activation　barrier　for　the　crucial　initial　step　of　cyclo-addition　become　more　favorable　on　the　bent　sections,　as　higher　pressure　is　applied.　These　values　are　also　found　to　be　linearly　correlated　with　the　pyramidalization　angle　θp,　which　can　be　further　related　to　external　pressure.　The　estimate　shows　that　it　is　easier　to　activate　tubes　with　larger　diameters　by　applying　pressure,　since　such　tubes　are　easier　to　deform.　Our　results　indicate　that　pressure,　as　an　easily　controlled　macroscopic　variable,　can　be　used　for　both　selective　adsorption　and　chemical　activation.　©　2005　Elsevier　Ltd.　All　rights　reserved.