A reflection-diffusion structure adopted for a light guide plate has a left wall, a right wall, and a bottom wall, which all are made of reflection materials. A top wall connects to the bottom wall, the left wall and the right wall, and a receiving cavity is formed by the left wall, the right wall, the top wall and the bottom wall. The top wall has a size-adjustable diffusion area made of a diffusion material. The left wall, the right wall, the top wall and the bottom wall are made integrally in one piece. The reflection material and the diffusion material are made integrally in one piece.