A series of ZrO2-Al2O3 composites with various compositions were synthesized by co-precipitation method, and the upconversion emission properties of the Er-Yb-Mo co-doped ZrO2-Al2O3 composites were investigated in the present paper. The green UC emission in the Er-Yb-Mo co-doped ZrO2 is an order of magnitude stronger than that of the Er3+ single doped sample. When Al2O3 mixed in ZrO2 crystalline lattice with the most solid solubility, the crystalline ZrO2-Al2O3 solid solutions is tetragonal ZrO2. The UC emission is enhanced 8 times in the ZrO2-Al2O3 solid solutions, which is due to the oxygen vacancies generated in the solid solutions, increasing the energy transfer from Yb3+-MoO2-4 to Er3+.