The　pull-in　problem　of　fixed-fixed　beam　structures　cannot　be　solved　analytically　due　to　its　nonlinear　from　the　large　deflection　and　the　electrostatic　driving　force.　So　to　understanding　the　characters　of　the　devices,　the　designers　have　to　resort　to　numerical　techniques.　Lack　of　an　accurate　model　for　predicting　pull-in　voltage　for　basic　micromachined　beam　structures　necessitates　the　clear　need　for　a　closed　form　expression　for　the　pull-in　voltage.　A　new　analytical　model　has　been　developed　based　on　Rayleigh-Ritz　method　for　determining　the　pull-in　voltage　of　an　electrostatically　actuated　micro-beam.　Three　new　parameters　with　the　dimension　of　voltage　square　have　been　introduced　for　considering　the　contribution　from　stretch　stress　gradient　nonlinear　stiffening,　bending,　and　the　residual　stress.　Thus,　the　model　can　takes　into　account　the　effects　of　axial　stress,　residual　stress,　stretch　stress　gradient　non-linear　stiffening,　and　fringing　fields　on　the　pull-in　voltage.　Modeling　result　has　been　compared　with　published　work　of　other　researchers　available　in　the　literature　and　FEM　simulation　results,　it　is　found　the　presented　model　estimation　results　agree　well　with　FEM　simulated　results　in　most　common　case.　©　2008　IEEE.