A novel compact microstrip bandpass filter using an open-loop resonator with radial stub is proposed, which has the advantages of compact size and sharp rejection in the stopband. One transmission zero is realized in the upper stopband in order to improve the selectivity of the passband. The transmission-zero frequency can be changed by altering the outer radius of the radial stub. A demonstration filter is designed and tested and the theoretical and experimental results are presented. © 2005 Wiley Periodicals, Inc. Microwave Opt Technol Lett 46: 387–389, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20995