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A tunable stripline resonator has been fabricated from YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) films deposited on single-crystal Y/sub 3/Fe/sub 5/O/sub 12/ (YIG) substrates. The films were deposited by laser ablation and employ buffer layers for epitaxial growth. The surface resistance of the YBCO on YIG has been measured independently and is 5/spl times/10/sup -4/ /spl Omega/ at 10 GHz and 77 K. The resonator is tuned with an external field that changes the permeability of the YIG. At 77 K we have observed tunablity of the fundamental (approximately 1.7 GHz in zero applied field) and the overtone resonances of the stripline. A mode of resonance frequency of 12 GHz has demonstrated tuning of 500 MHz for 500 Oe external field without appreciable degradation of the Q which is about 1200. For 2 kOe the tuning range is approximately 1 GHz. For higher applied fields the Q of the resonance degrades significantly presumably because of degradation of the surface resistance of the YBCO due to the applied field. The measured Q in zero applied field is about a factor of two to four lower than expected from the surface resistance of the film indicating that losses in the YIG may be affecting the performance.