Dark Mura phenomena which can happen at the region with high temperature gradient in a Notebook LCD Monitor using LED light source has numerically been studied. The calculation was conducted under the nearly realistic conditions by considering the anisotropic thermal properties of materials and the real dimensions of each component. The two performance indices of LED monitor, i.e., the maximum temperature and the spacial gradient of temperature were examined for the various shapes, lengths and thickness of heat sink plate. Calculated results give more reasonable temperature distribution comparing with experimental results.