A triangular red LED warning device comprises a compact box. A pair of oblique side red LED flashing boards, a base side red LED flashing board and a pair of supporting leg plates. The pair of oblique side red LED flashing boards are connected frictionally by a frictional tightening means at each one end to both ends of the base side red LED flashing board, and their each other ends are also connected detachably by means of an elastic connecting pin at neighborhood of the vertical angle, which is formed by the both oblique side boards. Thus, the triangular red LED flashing board can make various signal forms by frictional rotation and stoppage of the each oblique side board for a suitable degree against gravity, and can be freely folded to be received into a compact box after using.