The extraction-condensing cogeneration system is a popular technology for heat and power integration which can be used by petrochemical process. To compare with back pressure system, extraction-condensing system has better flexibility for process control. However, the thermal efficiency of extraction- condensing system could be affected by the amount of effective heat to process. If the effective heat to process and the plant power demand were not well designed, the cogeneration system may violate government regulation of ¡§qualified cogeneration system¡¨ by MOEA, or the system economics can not meet investor¡¦s requirement. From another point of view, if the cogeneration system bias original design operating condition or it has to run under low loading, the energy efficiency will move away from the target. A 94.9 MW extraction-condensing system of a petrochemical plant was selected as an example. For the purpose of data requisition, the author established a model to predict main steam flow, extraction steam flow, and power generation load. Moreover, a set of equations for the calculation of heat rate of turbine plant was developed. Besides, a Microsoft Excel calculation sheet was programmed to compute real time plant thermal efficiency. The actual operation data was compared with computer simulation. Results show (1) To meet the regulation, the process steam shall exceed 100 t/h with rated power generation. (2) For the minimum generator load (about 20 MW), the effective heat to process must exceed 78% in order to ensure a 52% overall thermal efficiency. (3) Low load means low thermal efficiency of this system. Some energy conservation ideas of this cogeneration system were assessed. Four ideas were presented, including (1) Increase boiler feed water temperature during low evaporation load. (2) Recovering of flash steam vented from blow down tank for the heating of boiler combustion air. (3) Control of cooling tower fans speed by using frequency inverter. (4) Utilization of hydraulic coupled forced draft fan. The total benefit of these energy conservation ideas is 2,546.44 kilo-liter fuel oil equivalent.