This investigation is a theoretical study of the thermodynamic performance of the organic Rankine cycle. A simplified analytical model is developed regarding the basic organic Rankine cycle by applying reasonable and realistic assumptions. Specifically, the analytical approach for the organic Rankine cycle efficiency uses the low cycle temperature, the high cycle temperature, the superheating degree, the liquid and vapor-specific heat capacities and the fluid latent heat at the high-temperature level. The calculated average deviation of the presented analytical model compared to the detailed thermodynamic one was calculated at 5.03% which is an acceptable value. Additionally, analytical approximations for the efficiency with regression models were created for three different working fluids named n-pentane, toluene and R600. The results of this work can be exploited for the quick and accurate analysis of organic Rankine cycles, and they can be used for the development of optimization models for reducing the computational time of the analysis.