In our previous work, we have shown that a single-domain YBa₂Cu₃O_x (YBCO) exhibits a low surface resistance. However, the second phase Y₂BaCuO₅ (211) precipitates inevitably as a result of peritectic reaction. These 211 particles are potential sources of RF losses, which need to be eliminated. In this study, a non-fluorine sol–gel synthesis was developed to deposit a YBCO thin film on the surface of single-domain YBCO. The deposited YBCO thin film entirely covered the 211 particles. The interface structure was studied by high-resolution transmission electron microscopy. The experimental results on sol–gel synthesis and thin film characterization are reported. Also discussed is the underlying mechanism of film growth on single-domain YBCO.