The　dymamical　behavior　of　MEMS　devices　is　strongly　affected　by　viscous　fluid　damping　effects　from　the　surrounding.　These　damping　effects　have　to　be　carefully　considered　for　the　design　and　optimization.　For　predicting　the　hydrodynamic　loading　of　an　electrostatical　device　in　viscous　fluids,　it　is　necessary　to　build　an　electrostatic-mechanical-fluidic　coupled　model,　where　the　flows　characteristics　were　described　by　Navier-Stokes　equations.　Comprehensive　analyses　of　the　static/dynamic　closing　behaviors　of　the　microswitch　in　viscous　fluids　were　performed.　Compared　with　published　work　of　other　researchers　available　in　the　literature,　the　numerical　model　was　validated.　After　that,　the　static/dynamic　closing　voltages　of　the　microswitch　in　the　air　and　deionized　water　were　calculated.　The　transient　behaviors　of　microswitch　under　different　action　voltages　were　also　simulated,　including　the　squeeze-film　damping　pressure　distribution　as　well.　The　results　show　that　the　difference　between　the　static　and　the　dynamic　pull-in　voltage　in　the　deionized　water　is　smaller　than　that　in　air.