In order to improve both operating voltage and memory coefficient of a protective layer for AC plasma display panels, controlled amount of alkaline earth oxide (CaO) was added to the pure MgO to replace the conventional pure MgO protective layer. The effects of CaO addition on both the electrical properties (V-f and V-s) and the microstructure of the Mg1-xCaxO thin films deposited on slide glass substrates by e-beam evaporation were investigated. Distinct changes in both peak intensity and diffraction peak positions of Mg-Ca-O films were observed with increasing [CaO/(MgO + CaO)] ratio owing to the strong atomic interaction between Mg and Ca. When the [CaO/(MgO + CaO)] ratio was increased to 0.1. the intensities of both (111) and (200) peaks were observed to increase with corresponding peak shift to lower diffraction peak positions as a result of the strong interdiffusion from the Ca and Mg atomic interactions. When the [CaO/(MgO + CaO)] ratio of 0.1 was used, the deposited films exhibited an enhanced operating performance with an improved memory coefficient. By adding a controlled amount of CaO. the deposited Mg-Ca-O films showed a firing voltage of 176 V that is lower than that of the conventional 100% MgO film.