Ruby (Al2O3:Cr) layer was successfully grown on a wall surface of an aluminum oxide crucible by isothermal evaporation of molybdenum trioxide (MoO3) flux. The growth was conducted by heating a mixture Of MoO3 and an oxide dopant (Cr2O3) at 1100 degrees C, for 5 h at this temperature without adding any reagent grade aluminum oxide. The ruby layers, obtained with a thickness up to 100-300 mu m, were transparent-red. The layers consisted of ruby crystals having flat surfaces. This technique was found to be a very suitable method for coating ruby layer on aluminum oxide materials.