The low-emissivity (low-e) coatings consisting of glass/ZnO/Ag were prepared and the microstructures of Ag thin films were investigated as a function of oxygen gas pressure during the ZnO underlayer deposition. It was found from X-ray diffraction measurement that the Ag thin films exhibited well-crystallization and (111)-preferred orientation when the ZnO underlayers were deposited in low oxygen gas pressure. Furthermore, glancing X-ray reflectivity measurement revealed that the surface roughness of Ag thin films decreased with decreasing oxygen gas pressure during the ZnO deposition. It can be said from these results that the highly crystallized and smooth Ag thin films grew on the ZnO thin films deposited in low oxygen gas pressure. Besides, it can be considered that the Ag thin films were heteroepitaxially grown using the smooth and highly crystallized ZnO underlayers which can be obtained under low oxygen gas pressure; as a result, the microstructures of Ag thin films should be improved.