In this paper, we propose a novel design of a UMTS diplexer using combined coplanar waveguide (CPW) and microstrip. The diplexer is composed of two bandpass filters, which termed as the Tx- and Rx-filter. The filter is composed of the lumped elements implemented by the CPW and microstrip structures. Zero has been implanted in each filter design for increasing the isolation between the Rx- and Tx-port. The two filters are connected in a parallel-cascade configuration. The microstrip is used to couple signals from/to the filters.