A lowpass-bandpass (LP-BP) diplexer with one lowpass channel (LPC) and one bandpass channel (BPC) is presented. The lumped-element dual-resonance resonator as common resonator is proposed to connect inductors, capacitors and LC resonator to constitute the desired channels. The LPC design is combined with parameters optimization and the lowpass transformation method, and the BPC design can be developed using the classical design theory of coupled-resonator filter. As an example, a 0.9 / 1.8 GHz LP-BP diplexer is designed and fabricated, which exhibits high return loss (RL), low insertion loss (IL), wide bandwidth (BW), high isolation and extremely compact size.