Expander is a key components in Organic Rankine Cycle (ORC) system. In order to improve the performances of single screw expander in large expansion ratio conditions and give consideration to normal conditions, a new idea called 'increasing the built-in volume ratio appropriately and converting single screw expander into double-stage machine in large expansion conditions by utilizing the discharge velocity of screw grooves was proposed. By implementing this idea, a new prototype of single screw expander was designed and manufactured. Then the prototype was integrated into an experimental ORC system with R123. Experiments were carried out to analyze the characteristics of the developed single screw expander and the ORC system. The results show that the maximum expander shaft power, shaft efficiency, isentropic efficiency, volume efficiency and expansion ratio were 8.35 kW, 56%, 73%, 83% and 8.5, respectively. In addition, under expansion losses seem to be eliminated by the new structure of the expander. It was also found that 12-17% of the expander power was consumed by the circulation pump, of which the measured efficiency was between 20% and 31%. A maximum ORC efficiency of 7.98% was achieved. (C) 2016 Elsevier Ltd. All rights reserved.