We　investigate　the　band　structure　of　narrow　armchair-edge　graphene　nanoribbons　(AGNRs)　under　tensile　strain　by　means　of　an　extension　of　the　Extended　Huckel　method.　The　strain-induced　band　gap　modulation　presents　asymmetric　behavior.　The　asymmetric　modulation　of　band　gap　is　derived　from　the　different　changes　of　conduction　and　valence　bands　near　Fermi　level　under　tensile　strain.　Further　analysis　suggests　that　the　asymmetric　variation　of　band　structure　near　Fermi　level　only　appear　in　narrow　armchair-edge　graphene　nanoribbons.