A U.L formulation was proposed for the nonlinear analysis of plane beam element with rigid arms. According to the characteristics of the motion and the deformation of a rigid-ended plane beam element, an explicit expression of non-linear tangent stiffness matrix was derived by U.L procedure. The deficiency of the reference that neglects the effects of element inner-force on additional term of the tangent stiffness matrix was improved to solve the divergence for strong nonlinear problems. An accurate deformation algorithm was presented,and a detailed calculation procedure was provided. A computer program was developed to analyze the geometrical nonlinear behavior of the plane beam with and without rigid arm. The results have demonstrated that the proposed innovative element and algorithm are efficient and accurate with satisfied convergence, and have promising applications in practical engineering projects.