Abstract: Acquiring natural-looking in-betweens is a fundamental shape interpolation problem in computer graphics. Several previous studies showed that as-isometric-as-possible interpolation is the key to natural and intuitive results. With this presumption, this paper describes a novel method for acquiring an isometric shape interpolation for given key-frame models. The technological and theoretical contributions of our method lie in the introduction of a new coordinate system, isometry-invariant intrinsic coordinates. This simplifies the nonlinear isometric interpolation into a simple linear algebraic problem. The method is shown to yield an effective isometric interpolation of the given key-frame models and is easy to implement. Experimental results confirm that the proposed method produces minimal metric distortion and exhibits reasonable efficiency.