In this article, a narrow-bandwidth duplexer with a triple-split ring resonator (T-SRR) using mu-negative metamaterial is presented. The T-SRR not only has negative permeability at the operating frequency band but also flexibility in size adjustment. The experimental results for insertion and return losses are 1.33 and 16.1 dB with a bandwidth of 10% at a RX center frequency of 8 GHz, and they are 1.27 and 16.8 dB at the same bandwidth at a TX center frequency of 10 GHz, respectively. (c) 2013 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:15371540, 2013; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.27656