A back light module is disclosed. The back light module comprises a frame, a reflective mask, a light guide plate, a linear light source and a baffle plate, wherein the reflective mask is installed in the frame. The linear light source is surrounded by the light guide plate and the reflective mask. The baffle plate is disposed between the linear light source and the light guide plate and the baffle plate is connected to the reflective mask and the frame. The baffle plate disposed between the linear light source and the light guide plate is adapted for absorbing impact force during the impact test and reducing the possibility of displacement of the light guide plate. Thus, the possibility of damage to the linear light source due to displacement of the light guide plate can be effectively reduced. Thus, the reliability of the notebook computer can be effectively promoted.