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In this paper, a duplexer using the open stub band-pass filter(BPF) with parallel coupled line is presented. The suggested duplexer is composed of distributed elements and planar substrate and it has characteristic of excellent isolation. The operating frequencies of the duplexer are respectively 5.8 GHz and 4.8 GHz at the RX and TX for C-band wireless communication systems. Also, the deviation frequency is 40 MHz and the bandwidth of filter is 10 %. The experimental results for insertion losses of filters are less than 2 dB at RX and TX, while the return losses are better than 15 dB.