The stress/strain behavior of the surround Cu stabilized YBCO coated conductors andits influence on critical current were precisely investigated. The internal strainexerted on the superconducting YBCO layer was determined at 77 K by using aneutron diffraction technique at JAEA. The initial compressive strain decreasedduring tensile loading and changed to a tensile component at the force free strain (Aff), where the internal uniaxial stress becomes zero in the YBCO layer. TheAff was evaluated to be 0.19–0.21% at 77 K. The critical current measurements werecarried out under a uniaxial tensile load at 77 K. The strain dependence revealed acharacteristic behavior, where a maximum was observed at 0.035%. Thus it was madeclear that the strain at the critical current maximum does not correlate withAff for YBCO coated conductors.