The ternary Zn1-xCdxO (0 <= x <= 1) alloy thin films were deposited on quartz substrate by the direct current (dc) reactive magnetron sputtering method. As x varied from x = 0 to 0.5, the Zn1-xCdxO thin films had a hexagonal wurtzite structure of pure ZnO and highly (0 0 2) preferred orientation. The optical band-gap energies of the Zn1-xCdxO thin films were tuned from 3.24 eV at x = 0 to 2.04 eV at x = 0.5. But for the samples with x >= 0.8, the Zn1-xCdxO thin films had a rock-salt structure of pure CdO and highly (2 0 0) preferred orientation. Correspondingly, the optical band-gap energies of the Zn1-xCdxO thin films decreased from 2.35 eV at x = 0.8 to 2.22 eV at x=1, which has a little change. The variations of the band-gap energies for the thin films with hexagonal wurtzite structure were analyzed from the viewpoint of band structure. (C) 2015 Elsevier Ltd. All rights reserved.