We have used a parallel‐plate resonator technique to measure the microwave surface resist‐ anceRsof YBa2Cu3O7−x(YBCO) films on buffered ceramic substrates at around 10 GHz, and studied the correlation between theirRsand materials properties. A 0.4‐&mgr;m‐thick YBCO film (with an in‐plane mosaic spread of 7°) grown on a polycrystalline alumina substrate with an ion‐beam‐assisted‐deposited yttria‐stabilized zirconia buffer layer showed anRsof 1.89 m&OHgr; at 76 K and 0.21 m&OHgr; at 4 K. We have observed a strong correlation between theRsof the samples and the in‐plane mosaic spread of the YBCO films. This correlation can be explained qualitatively in terms of a simple model in which the weak links between the grains of the YBCO film form an electrical network of Josephson junctions.