This paper conducts numerical investigations for a 15% thickness Co-Flow Jet (CFJ) airfoil performance enhancement, which includes the variation of lift, drag, and energy expenditure at Mach number 0.03, 0.3, and 0.4 with jet momentum coefficient Cμ = 0.08. The angle of attack(AoA) varies from 0° to 30°. Two-dimensional simulation is conducted using a Reynolds-averaged Navier-Stokes (RANS) solver. A 5th order WENO scheme for the inviscid flux and a 4th order central differencing for the viscous terms are used to resolve the the Navier-Stokes equations. Turbulence is simulated with the one equation Spalart-Allmaras model. The study shows that at constant Cμ, the maximum lift coefficient is increased with the increasing Mach number due to the compressibility effect. However, at M=0.4, the airfoil stalls with slightly lower AoA due to the appearance of strong λ shock wave that interrupts the jet and trigger boundary layer separation. The drag coefficients vary less with the Mach number, but is substantially increased at Mach 0.4 when the AoA is high due to shock wave-boundary layer interaction and wave drag. The power coefficient is decreased when the Mach number is increased from 0.03 to 0.3. This is again due to the compressibility effect that generates stronger low pressure suction effect at airfoil leading edge, which makes the CFJ pumping easier and require less power. For the same reason of shock appearance at M=0.4 when the AoA is high, the power coefficient is significantly increased due to large entropy increase. Overall, the numerical simulation indicates that the CFJ airfoil is very effective to enhance lift, reduce drag, and increase stall margin with high Mach number up to 0.4 at low energy expenditure.