Abstract: Through an example of solving the rotation angle of truss members, the standard solution and two misconceptions of using the unit load method to solve the rotation angle of truss member are introduced. The causes and solutions of the two wrong solutions are analyzed utilizing the basic assumptions of structural mechanics and the basic concepts of unit load method. It shows the importance of the basic assumptions and basic concepts of the theories and methods, and it can be used as reference for teaching and learning of structural mechanics and relative courses.