Abstract: Organic Rankine cycle (ORC) is regarded as a promising technique to recover energy from the low and medium-grade heat. The exergy destruction of the ORC system is mainly caused by poor temperature match in heat transfer processes of the evaporator and condenser. Organic flash cycle (OFC) with zeotropic mixture working fluid has the potential to optimize the temperature match both in the evaporator and condenser due to sing-phase liquid heat transfer and condensation temperature glide. However, the throttling processes introduce the irreversibility. To analyze whether the OFC system can provide a net gain in the exergy efficiency, two other optimized cycles: modified OFC (MOFC) and modified 2-phase OFC (MPOFC), were compared in the present work. The effect of the concentration of mixtures on the exergy efficiency of the OFC system was calculated. The results show that MOFC and MPOFC further improved the exergy efficiency by reducing the exergy loss. On a proper mixture concentration, the MOFC and MPOFC could achieve higher exergy efficiency than the ORC. The maximum exergy efficiency of the MPOFC reaches 58.50% at the neopentane/cyclopentane concentration of 0.6/0.4.