We　quantified　the　size-dependent　energy　bandgap　modulation　of　ZnO　nanowires　under　tensile　strain　by　an　in　situ　measurement　system　combining　a　uniaxial　tensile　setup　with　a　cathodoluminescence　spectroscope.　The　maximal　strain　and　corresponding　bandgap　variation　increased　by　decreasing　the　size　of　the　nanowires.　The　adjustable　bandgap　for　the　100　nm　nanowire　caused　by　a　strain　of　7.3%　reached　approximately　110　meV,　which　is　nearly　double　the　value　of　59　meV　for　the　760　nm　nanowire　with　a　strain　of　1.7%.　A　two-step　linear　feature　involving　bandgap　reduction　caused　by　straining　and　a　corresponding　critical　strain　was　identified　in　ZnO　nanowires　with　diameters　less　than　300　nm.　The　critical　strain　moved　toward　the　high　strain　level　with　shrunken　nanowires.　The　distinct　size　effect　of　strained　nanowires　on　the　bandgap　variation　reveals　a　competition　between　core-dominated　and　surface-dominated　bandgap　modulations.　These　results　could　facilitate　potential　applications　involving　nanowire-based　optoelectronic　devices　and　band-strain　engineering.