In this paper, RF MEMS capacitive switch with a novel metal/dielectric (Au/SiN) composite beam on highly resistive silicon substrate is presented. The low-stress SiN under the Au electrode layer is adopted due to its excellent dielectric and mechanical characteristic. Four beam structures are designed, and mechanical/electrostatic and high frequency analysis tools are used to explore their performance. It can be seen from the analysis that satisfying switching property can be obtained from careful choice and optimization of the structure. Besides, the switch has shown excellent radio frequency (RF) performance. In addition, the composite beam, which is stiffer than pure metal ones, may not only provide good DC insulation and RF isolation augmentation but also theoretically help to reduce the danger of beam stiction. With different structures, this switch can meet the requirement of a wide variety ofhigh frequency applications. These features made this micro switch an optimal solution for mobile or fixed RF applications, such as in the construction of millimeter wave phase shifter MMIC and switched tunable filters, for wireless telecommunication or radar use.