Microstrip transmission lines in the form of an open-ended half-wavelength resonator were fabricated by depositing YBa₂Cu₃O _x thin films on MgO substrates using the pulsed laser deposition method. All YBa₂Cu₃O _x thin films had c-axis orientation. The effects of etching time and thickness on the performance of the microstrip line resonator were investigated. As the etching time increased, the loaded quality factor decreased and the surface resistance increased due to the undercut and the increase in roughness of the etched surface. The quality factor and the surface resistance showed a strong dependence on the film thickness. The superconducting properties and the microwave characteristics correlated well with the microstructure.