There are a large quantity of low grade waste heat which is generally less than 90℃ in some industrial plants. How to efficiently utilize this part of heat during the nonheating season has become an interesting and significant research topic during the last decade. In this study, a simulation model of an organic rankine cycle (ORC) power generation system is developed. Using the waste water of a steel mill as the heat source of the ORC system, the most efficient available working medium named R245fa is found under the designed working condition. The influence of the temperature variation of the heat source on the performance of the entire system is also investigated in this paper. The result shows that the electric power output and the system thermal efficiency can be increased with the increase of the heat source temperature. According to the analysis of this investigation, when the heating temperature increases from 70 ℃ to 90 ℃, the electric power increases from 212KW to 805KW and the thermal efficiency increases from 2.14% to 7.76%.