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A NiO surface produced by surface-oxidation epitaxy (SOE) method was flattened by a mechanical polishing technique. The surface roughness of NiO was reduced in this manner to about 5 nm. In addition, thin perovskite films were deposited on the polished NiO as a new cap layer. Consequently, Jc of the YBCO films formed on BaSnO3/NiO/Ni substrate by pulsed laser deposition reached 0.45 MA/cm2 (77 K, 0 T), and Jc of YBCO/BaZrO3/NiO/Ni increased up to 0.89 MA/cm2 (77 K, 0 T). Development of Jc is due to the reduction of superconducting weak coupling by flattening of NiO surfaces and the improvement of crystal orientation of YBCO films by the use of perovskite cap layers.