Nanodiamond with dielectric strength greater than 2MV/cm was grown by microwave plasma enhanced chemical vapor deposition and used as a leaky dielectric film for RF MEMS capacitive switches. Nanodiamond films grown by MPECVD were compared with Si3N4 films deposited by RFPECVD by means of transient current measurements. Nanodiamond was characterized by SEM, AFM and Raman spectroscopy for correlation with switch performance. The DC resistivity of nanodiamond was found to be lower than that of Si 3N4 by 3 to 6 orders of magnitude. The discharge time constant of nanodiamond was, therefore, much smaller than that for Si 3N4. Extended DC bias was applied to enhance dielectric charging and demonstrate the superior performance of nanodiamond to that of Si3N4 by showing the much better persistence of capacitance-voltage characteristics of nanodiamond after being subjected to extended DC bias. Superior nanodiamond characteristics were further demonstrated at an elevated temperature at (150°C) and at the dry ice temperature (-79°C).