We proposed a phenomenological model for the morphology evolution of oxide scales thermally growing on metals and alloys during oxidation. This model is based on two well-known experimental facts: (1) the growth rates of the crystalline facets of oxide might differ from each other; (2) the nucleation density of oxide might vary from system to system of alloy/oxide. A two-dimensional computer simulation was formulated. The simulations predicted the formation of equiaxial, needle-like, stump-like and flower-like morphology of oxide scales on alloys. The simulation results showed that equiaxial oxide scales formed when the nucleation rate was high and the growth rates of facets did not differ too much from each other, while needle-like scales formed when the growth rates of a pair of facets was much higher than those of other facets. It was also shown that the stump-like scales usually accompanied the flower-like ones when parts of facets grew at high rates. The numerical results were in good agreement with the morphologies of oxide scales on several sputtered CoCrAlY alloys.