This letter presents a reliable and very easy method for selective growth of polarity controlled ZnO films on (0001) Al2O3 substrates by plasma-assisted molecular-beam epitaxy. Cr-compound intermediate layers are used to control the crystal polarity of ZnO films on (0001) Al2O3. ZnO films grown on rocksalt structure CrN/(0001) Al2O3 shows Zn polarity, while those grown on rhombohedral Cr2O3∕(0001) Al2O3 shows O polarity. Possible interface atomic arrangements for both heterostructures are proposed.