The Mars Wind Tunnel at Tohoku University is a low-density tunnel that can simulate Martian atmospheric flight condition. Up to the present, this tunnel has been operated using air as the working gas. To expand the capability of this tunnel and to study the effects of compressibility on airfoil performance at low Reynolds number, a modification has been made to allow the tunnel to be operated using CO2, that is the main constituent of the Martian atmosphere. In this study, the operational characteristics as well as the flow quality in the test section are investigated in CO2 mode and compared with the results obtained in air mode. It is found that a higher Mach number can be achieved in CO2 mode than in air mode as the speed of sound decreases with the molecular weight. No negative effect of CO2 such as freezing inside the ejector nozzles has been observed over the entire operational conditions of the tunnel. Mach number profile is found to be uniform over a wide range of Reynolds number except inside the boundary layers on the walls. The static pressure gradient along the test section is almost zero at the total pressure of lkPa. These results prove that the Mars Wind Tunnel has a capability to simulate the real Martian atmospheric condition.