A light guide plate (LGP) includes a positioning block that includes a positioning block body and a liquid filled and hermetically sealed in a receiving compartment formed in the interior of the positioning block body. The liquid is expandable with a drop of temperature so as to increase a volume thereof and thus enlarge a size of the positioning block body through elasticity of the positioning block body. In this way, the LGP positioning block is adjustable with the variation of the surrounding temperature so as to achieve effective positioning of the light guide plate and providing high reliability of a liquid crystal display device including the light guide plate.