The　silicon　nitride　thin　films　are　deposited　on　polyimide　sacrificial　layer　by　PECVD.　The　influence　of　the　process　parameters　such　as　temperature,　power,　the　ratio　flux　of　reactant　on　the　residual　stress　and　other　characteristics　of　silicon　nitride　thin　films　are　discussed.　The　optimum　process　condition　is　obtained,　and　the　silicon　nitride　thin　films　deposited　by　the　process　condition　have　been　successfully　applied　in　RF　MEMS　switch.