Abstract: Insertion loss is an important performance indicator for radio frequency micro-electro-mechanical systems (RF MEMS) switches. The RF MEMS capacitive switch is a kind of RF MEMS switch which is suitable for high frequency use. Its loss mechanisms are investigated in this paper. Four main parts contribute to the RF MEMS capacitive switchs insertion loss: conductor loss of the signal lines, substrate loss, radiation loss, and MEMS bridge loss. Loss models are built and calculated, and numerical calculation results agree well with simulation results. In addition, influence factors of insertion loss are analyzed, and results show that high resistivity substrate, conductor width around 200 m, smaller conductor thickness and smaller up-state capacitance can help lower the insertion loss.