A novel double-layer suspended stripline (DSSL) resonator with high unloaded quality factor is proposed. The DSSL resonator consists of a thin substrate with metal strips on both sides connected by a metallic via hole suspended in the centre of an air-filled cavity, which leads to a much higher unloaded quality factor than the conventional single-layer suspended stripline resonator with an identical resonant frequency. The resonance characteristics of the DSSL resonator are parametric analysed. For demonstration, a compact diplexer for base-station application with channel centre frequencies at 1.95 and 2.14 GHz and absolute bandwidths of 60 MHz is designed, fabricated and measured. The synthesised, simulated and measured results show a good agreement.