The effective utilisation of low and medium temperature energy is one of the key solutions to alleviate energy shortages. In the past decades, Organic Rankine Cycle (ORC), which can convert renewable energy into electrical power, has received widespread attention and research because of its feasibility and reliability. Meanwhile, group contribution methods (GCMs), which are based on the additivity of groups, have been widely used to estimate the properties of compounds only from their molecular structures. Therefore, a group contribution model for simulating the thermodynamic processes and performance of basic ORC has been developed in this paper. Four working fluids R123, R245fa, R600 and R601a were selected to be used for the ORC, in which the condensation temperature remained constant at 300K while the evaporation temperature varied from 350K to 400K. Their normal boiling temperatures, critical parameters, saturated liquid densities and ideal gas heat capacities were obtained via the existing GCMs. Other properties needed in the analysis of ORC characteristics were calculated by the thermodynamic relationships with the estimated properties of GCMs. Four thermodynamic processes and the cycle efficiency of ORC have been determined by the proposed model while the estimation accuracy of the model was validated by comparing with the calculations of NIST. The average absolute deviations for the processes including compression, evaporation, expansion and condensation, and efficiency of the considered ORC were 8.54%, 2.24%, 2.23%, 2.28% and 1.49% respectively. It was concluded that the proposed model can estimate the ORC characteristics of any pure working fluid correctly based only on its molecular structure.