A frame structure used in a backlight module is disclosed. The frame structure includes an upper frame and a lower frame. By combining the T-shape hook of the upper frame and the corresponding groove of the lower frame, the components of the backlight module, such as the linear light source, the light source fixer, the light guide, the upper diffusing plate, the reflective plate, the lower diffusing plate, and the prism plate can be well assembled. Therefore, an improved positioning and fixing efficacy is provided.