An f0/2f0 (frequency ratio of two) microstrip diplexer with simple circuit configuration as well as low and wideband insertion-loss characteristics is proposed. It is a parallel combination of a coupled line for f0 port and a wave-trap circuit composed of a transmission line and an open stub for 2f0 port. All the lines and stub have a quarter-wave length for f0. Matching circuits are not needed. Circuit and electro-magnetic simulation results prove that the proposed f0/2f0 diplexer exhibits well-balanced properties of insertion loss (IL), IL bandwidth, and isolation, as compared to conventional simple f0/2f0 diplexers composed of two wave-trap circuits or two coupled lines. The proposed diplexer is fabricated on a resin substrate in a microstrip configuration at frequencies of f0/2f0=2.5/5 GHz. Measured results are in good agreement with simulations and support the above conclusion. The proposed diplexer exhibits ILs of 0.46/0.56 dB with 47/47% relative bandwidth (for f0/2f0), which are lower and wider than f0/2f0 diplexers in literatures at the same frequency bands.