In this paper, the modeling and design of high-Q multilayer passives and multilayer diplexer for GSM/DCS applications designed and fabricated using these passives have been investigated.. To miniaturize the system, configurations of inductor and capacitor have involved a square spiral structure and a vertically-interdigitated capacitor similar to 3D interdigital structure, respectively. Multilayer diplexers for GSM/DCS applications were designed and fabricated to apply high-Q multilayer passives to practical systems, which were designed by the proposed structural and equivalent circuit model. LPF for GSM band had the passband insertion loss of less than 0.55 dB, the return loss of more than 12 dB, and the isolation level of more than 26 dB by locating attenuation pole at 1800 MHz. HPF for DCS band had the passband insertion loss of less than 0.82 dB, the return loss of more than 11 dB, and the isolation level of more than 38 dB by locating attenuation pole at 930 MHz.