This　paper　presents　a　mathematical　model　of　torque　for　Single　screw　expanders　(SSEs).　Instantaneous　torque　and　torque　ratio　were　analyzed　and　discussed.　The　periodic　variation　of　instantaneous　torque　is　the　same　for　different　inlet　pressure　levels.　The　torque　ratio,　with　its　value　close　to　1,　is　independent　of　the　inlet　pressure　of　SSE.　An　experimental　system　was　established　to　measure　the　torque,　power　and　shaft　efficiency　of　the　self-developed　SSE　prototype,　and　results　were　used　to　validate　the　model.　Comparison　shows　that　the　difference　between　calculated　and　experimental　torque　values　is　small　(6.58　N.m　to　7.55　N.m).　The　calculated　and　experimental　output　power　is　similar,　with　a　difference　of　2.07　kW　to　2.37　kW.　Therefore,　the　proposed　model　can　be　used　to　estimate　the　torque　and　output　power　of　SSEs.