An excellent correlation exists between the firing voltage and the secondary electron emission coefficient (gamma) in MgO films used as protective layers in ac plasma display panels. In order to make a new protective layer with a high secondary electron emission coefficient and with a reduction in the demanding voltage required for discharge, we have studied the effect of CaO addition on the secondary electron emission characteristics of MgO film. Mg1-xCaxO films were prepared by the e-beam evaporation method. As a large number of Ca atoms were substituted for Mg atoms, lattice distortion occurred in MgO films (NaCl crystal structure) in accordance with the increase in the lattice parameter. When the cation ratio in films was 0.088, relaxation of compressive stress seems to be caused by crack formation in the film. The density of pure as grown MgO was 3.214 g/cm(3) which was 88% of the bulk MgO's density. The density of the Mg1-xCaxO films gradually increased by about 14% with CaO addition to MgO. Secondary electron emission yield of pure MgO film was 0.33. Addition of a controlled amount of CaO to MgO resulted in a rapid increase in the secondary electron emission yield. The film with a [Ca/(Mg+Ca)] ratio of 0.134 yielded a secondary electron emission coefficient as high as 0.58.