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.