Electric birefringence of nematic liquid crystals in the isotropic phase and the wall effects were analyzed upon a basis of the Landau-de Gennes theory. Obtained phase diagrams explain the electro-optic characteristics of novel liquid crystal displays using electric birefringence very well and the analytical results of wall effects provide qualitative explanation of the effects of substrate surface treatment on the operation voltage. The present analysis is expected to be useful to improve the display performance and optimize the display panel design.