Abstract: Several Authors have presented numerical results which demonstrated the stiffening of 8-node and 12-node quadrilateral isoparametric elements when distorted from a rectangular shape. In consequence the error introduced by using distorted 8-node or 12-node quadrilateral isoparametric element is significant. The results obtained by distorted elements are essentially consistent with those obtained by undistorted elements if 9-node element is used as quadrilateral isoparametric element and Lagrange polynomials as interpolation functions. Numerical results show that 9-node isoparametric element is superior to 8-node isoparametric element, 12-node isoparametric element and quadrilateral element consisting of two or four linear strain triangles.