Abstract: This paper provides unified sign convention for the internal and external forces of rod on the basis of a local coordinate system.Three types of fundamental deformations in the internal forces of rod are analysed with the same method,and this method can be applied expansively in internal forces analysis of a plane rigid frame.The method has merits-a distinct concept,a handy calculation,the same working procedure.It is convenient for teachers,students and engineers to learn and apply it,and surmount difficulties on varied rules.