Flexible hinge transfers motion and force by deformation of flexible element, the stiffness of which made by metal material can be calculated on the premise of Hooke’s law. While the stress-strain relation of hyperelastic material is unconformable to Hooke’s law, it must be derived by stress-energy functions. To analysis the stiffness of flexible hinge made by hyperelastic material, a notch flexible hinge was proposed. Approximate formula for stiffness of such hinge was deduced and its scope of application was analyzed. Analysis result comparison between finite element analysis model and approximate formula shows consistency. Flapping flight is ubiquitous in nature and all birds and insects have compliant thoraxes for activating wings to flap. Flexible hinge using in the construction of mechanism offered improvements over rigid hinge on some aspects of large range of motion, long life and saving in power. Application in compliant thorax of micro aero vehicle with flapping wings was realized.