Studies on plasma spraying of zircon (ZrSiO4) have been carried out by the authors as one of the candidates for an environmental barrier coating (EBC) application, and had reported that substrate temperature is one of the most important factors to obtain crack-free and highly-adhesive coating. In this study, several amount of yttria were added to zircon powder, and the effect of the yttria addition on the structure and properties of the coatings were evaluated in order to improve the stability of the zircon coating structure at elevated temperature. The coatings obtained were composed of yttria stabilized zirconia (YSZ), glassy silica, while the one prepared from monolithic zircon powder composed of the metastable high temperature tetragonal phase of zirconia and glassy silica. After the heat treatment over 1473K, silica and zirconia formed zircon in all the coatings. However, the coatings with the higher amount of yttria had less amount of zircon formed. This resulted in the less open porosity of the coating at elevated temperature. These yttria added coatings also showed good adhesion even after the heat treatment, while monolithic zircon coating had peeled off.