Abstract: Organic Rankine cycle（ORC） is a promising technology for converting biomass energy to electricity.For a biomass direct combustion system with a 7.9 MW boiler,which used wheat straw as fuel,and took heat conduction oil as intermediate heat transfer medium,the operating parameters of the ORC working fluids such as decane,toluene,cyclohexane,octamethyltrisiloxane（MDM）,octamethylcyclotetrasiloxane（D4） and hexamethyldisiloxane（MM） were optimized.The optimum turbine inlet temperature decreases as the critical temperature of the working fluid increases by pinch analysis.The turbine inlet and outlet pressures of decane,MDM and D4 are much lower;thus,turbines using decane,MDM or D4 as the working fluid must be larger.The extraction coefficients of MDM,D4 and MM are higher since the exhaust temperatures are greater than 200 ℃.The ORC thermal efficiencies are greater than 25% for a condensation temperature of 70 ℃,the power outputs are greater than 1 600 kW and the thermal loads are greater than 4 800 kW.For a condensation temperature of 100 ℃,the thermal efficiencies are greater than 20.7%,the power outputs are greater than 1 300 kW and the thermal loads are greater than 5 100 kW.The energy utilization factors are higher than 82% for both conditions,while the exergy efficiencies are greater than 23%,most of the exergy destruction occurs in the boiler.The exergy efficiency slightly decreases as the condensation temperature decreases.The thermal performance characteristics of decane,toluene and cyclohexane are slightly better than for the siloxanes.