The　scroll　expander-generator　is　suitable　to　be　used　in　the　small-scale　organic　Rankine　cycle　system　to　generate　electricity.　In　this　article,　a　mathematical　model　has　been　developed　which　includes　thermodynamic　model,　mechanical　model,　and　load　resistance　model.　The　stable　rotating　speed,　volumetric　efficiency,　and　isentropic　efficiency　influenced　by　the　suction　pressure,　discharge　pressure,　and　generated　current　have　been　analysed.　The　results　show　that　the　stable　condition　of　scroll　expander-generator　is　determined　by　the　driven　and　resistance　moments.　The　transient　rotating　speed　increases　with　the　running　time　until　the　machine　runs　stably　in　the　starting　process.　The　scroll　expander-generator　is　accelerated　or　decelerated　when　the　suction　pressure,　discharge　pressure,　and　generated　current　are　changed.　The　stable　time　increases　with　the　variation　of　suction　pressure　either　in　the　acceleration　or　deceleration　process.　The　stable　rotating　speed　and　volumetric　efficiency　increase　with　the　suction　pressure　but　decrease　with　the　discharge　pressure　and　generated　current.　The　isentropic　efficiency　increases　with　the　suction　pressure　but　decreases　with　the　generated　current.　A　maximum　isentropic　efficiency　is　found　with　the　increase　of　discharge　pressure.