This paper presents novel 3D heterogeneous integrations using MEMS Devices for RF applications. We propose a 3D heterogeneous integration method that combines the advantages of LTCC, passive integration, and MEMS technologies. The basic concept is to form a large-size LTCC wiring wafer and then to form high-Q passives or MEMS filters directly on the wafer surface. Other functional devices such as ICs, SAWs, and MEMS switches are mounted above the surface-formed devices. A miniaturized duplexer consisted of IPD, SAW, and film bulk acoustic resonator (FBAR); and a next generation duplexer module consisted of an MEMS tunable filter and a piezoelectric transducer (PZT)-actuated RF MEMS switch were constructed to demonstrate its feasibility and effectiveness.