A warning triangle has a simplified structure and is operable in accordance with the requirement of energy saving and manufactured at lower cost. The warning triangle structure includes a first board body, a second board body and a third board body, which are assembled with each other. Each of the board bodies has a first face and a second face, which together define an internal cavity. At least one light source and a light guide strip for guiding light are disposed in the cavity. The second and third board bodies are pivotally connected with movable stands for supporting the warning triangle in use. The second and third board bodies can be selectively collapsed onto the first and second faces of the first board body to minimize the volume of the warning triangle.