They concluded that circulating refrigerant mass and composition are the primary controlling factors with regard to transient capacity oscillations. In recent years, vapor injection cycle has been studied by different research groups due to its prominent performance enhancement compared to a convention single-stage cycle (Xu et al., 2011). Wang et al. (2009) tested the flash tank cycle at "18 #C and found the maximum heating capacity and COP improvements to be 33% and 23%, respectively. Ma and Zhao (2008) investigated the vapor injection heat pump cycle employing a flash tank coupled with a scroll compressor. The system demonstrated sufficient heating capacity of 8.15 kW at a condensing temperature of 45 #C and an evaporating temperature of "25 #C. They concluded that the system could provide sufficient heating in severely cold regions. Bertsch and Groll (2008) tested a two-stage heat pump system at low ambient temperature of "30 #C. The heating COP was found to be 2.1, and the heating capacity almost doubled as compared to a single-stage cycle. However, most of the work done on a vapor injection cycle was focused on utilizing R22 and R410A. With the trend of phasing out R22 and R410A, it’s necessary to investigate new refrigerant candidates for a vapor injection cycle. As can be seen in research efforts in the literature, although R32 has been studied experimentally and theoretically, all works have been conducted on a conventional single-stage vapor compression cycle. Employing refrigerant R32 in a vapor injection two-stage system is worth for investigation, since there was no open publication on such research effort. This paper focuses on comparing the performance between R410A and R32 in a vapor injection system with a flash tank at different operating conditions.