Aerodynamic characteristics of airfoils in a propeller slipstream were investigated at low Reynolds number. The effects of a propeller slipstream on control surface effectiveness were clarified by aerodynamic force measurement and the Particle Image Velocimetry (PIV) measurement. The tests were conducted using the small low-turbulence wind tunnel in the Institute of Fluid Science, Tohoku University, at the chord Reynolds number of 3.0×104. The airfoils tested were the Ishii and T1-2 airfoils, the two main candidates for the Japanese Mars airplane. Two configurations were used in the testing, one configuration with a propeller set in front of the wing and one configuration without a propeller to be used as a baseline. For the case without the propeller, the Ishii airfoil control surface effectiveness increased according to the deflection angle. However, the effectiveness of the T1-2 airfoil did not increase at certain angles of attack. When the propeller was set, the effectiveness increased according to the deflection angle for both airfoils. These results indicate that a propeller would have a positive effect on controllability of a Mars airplane.