Improving the efficiency of trans-critical CO2 heat pump systems becomes one of the investigative focuses of the development of refrigeration technologies. The use of expander can improve the efficiency of trans-critical CO2 cycles, and make it near to that of traditional refrigeration systems. On the basis of the previously developed single-cylinder rolling piston expander, a new two-rolling piston expander is designed and produced. The design process and its working principles are introduced, and a trans-critical CO2 water-water heat pump system is used to test its efficiency. Results of the tests show that under working conditions the rotating speed of the expander ranges from 850 to 1000 rpm, and its efficiency ranges from 28% to 33%. Analysis of the problems that appeared in the expander is made to further improve its efficiency. (C) 2013 Elsevier Ltd. All rights reserved.