Abstract: Through coordinate transformation corresponding to the isoparametric element theory, calculation of an abnormal element in Cartesian coordinate system is converted to a regular element in natural coordinate system.Applying this finite element technique to nonlinear ful-l range analysis theory of arbitrary reinforced concrete cross-sections, useful expressions of cross-section s strain field and stiffness matr ix in natural coordinate system are derived.In square element, procedure of carrying out Gauss integral to obtain every component of the stiffness matrix is proposed.This method makes numerical analysis methods of various shapes uniformed with one model.A comparison between the numerical analysis by arc-length method and the test results shows good agreements in ultimate loads