This paper numerically studies the Reynolds number effect for a 2D Co-Flow Jet (CFJ) airfoil and a 3D wing at freestream Mach number of 0.46. The Reynolds averaged Navier-Stokes equations solver with Spalart-Allmaras one-equation turbulence model is utilized for the simulation. The Reynolds number is decreased from 4.1 ×106 to 0.18 ×106 to mimic the altitude change from 10000m to 30000m. The results show that, at Reynolds number of 4.1 × 106, the best CFJ airfoil corrected aerodynamic efficiency ((CL /CD )c ) of 81.64 occurs at Cµ of 0.03 at AoA of 6◦ . When the Reynolds number is reduced to 0.18 × 106, the (CL /CD )c has a 39.5% decrease. It is because of the dramatic increase of the viscous drag by 66.2%. However, the lift coefficient drops only by 5.1%. The second reason for the decrease of the CFJ airfoil corrected aerodynamic efficiency is that the power coefficient P c is increased significantly by 85% with the decrease of the Reynolds number. It is caused mostly by the increased total pressure ratio of CFJ pumping due to higher loss. The 3D wing with an aspect ratio of 20 based on the same 2D airfoil is also studied for the same Reynolds numbers and freestream Mach number. Similar to the 2D cases, the wing at low Reynolds number suffers significantly increased viscous drag and energy loss of the CFJ.