Organic Rankine cycle (ORC) is recently receiving increasing attention and regarded one of the most promising low-grade heat-to-power technology. Compared to the conventional high-grade fossil energy-to power technologies, ORC is much more sensible to the environmental temperature variation due to the low temperature of heat sources. A conventional ORC is typically designed under a given condition and mostly operate under off-design conditions, resulting in low annual average performance. This study proposes a novel zeotropic ORC in which the mixture composition can be tuned in response to the environmental temperature variation using liquid - separation condensation. A thermodynamic optimization model is formulated to optimized the mixture composition according to the environmental temperature. A concept design of liquid-separation condensation -based composition tuning system is conducted. The results show that the proposed ORC can significantly improve the overall efficiency without extra cost increment, which can be improved by up to 9.15 % compare to the conventional ORC for 100 oC heat source.