A novel electrical lumped model for an RF MEMS capacitive switch which is imple- mented on a semi-suspended coplanar-waveguide is proposed and studied. The quality factor is mathematically studied taking the effects of air-gaps and substrate resistance into consideration. Results indicate that increasing the equivalent resistance of transmitting line and the switch, series resistance, in down-state has negative effect and can considerably decrease the quality factor. Increasing the capacitance of the switch also has a negative effect on the quality factor. Increasing the substrate resistance improves the quality factor. Lower capacitance can allow the switch to operate with a wider bandwidth of high quality factors and larger capacitance can make the bandwidth narrower.