A “Thick film Electroluminescence (EL) device” has advantages in its thin structure, light weight, uniform lighting, and mechanical flexibility (bending). From these advantages, the thick film EL can be regarded as the optimum light source for recent cars which require a high packaging density consisting of various components to be mounted in a small space.EL is now being used for light sources for both interior and exterior light sources for automotive applications utilizing its features. However, it has not yet reached a satisfactory level of performance and had only limited application areas because the thick film EL device up to today has a tendency that its luminous intensity rapidly decreases when used in a high temperature and high humidity environment.This time, we prototyped an EL device utilizing a transparent conductive film which can give moisture proof effect and uniform luminous intensity, and we evaluated and analyzed its performance. Consequently, we confirmed that the degradation of luminous intensity is caused by moisture invasion from outside EL to resolve the phosphor.Based on the above findings, we have developed a new thick film EL device with new structure and materials which extends its life twice as long as conventional one under a high temperature and high humidity environment. Also, a high performance inverter has been developed which controls its output along with the impedance variation of the EL device over its life in order to satisfy automotive specification.