The objective of the present study is to modify mixing and heat transfer in impinging jets using a single-frequency excitation imposed at the jet exit. The excitation frequency is selected to be $St_{ heta}=f{ heta}/U_{J,max}=0.017$ where ${ heta}$ is the jet-exit momentum thickness and $U_{J,max}$ is the jet-exit maximum velocity. In free jets, this excitation results in turbulence suppression in a downstream location. On the other hand, in impinging jets, the effect of excitation significantly depends on the distance (H) between the jet exit and the impinging wall. For large H (e.g. H/D=10, D is the jet exit diameter), the Nusselt number near the stagnation point ($Nu_{stag}$) decreases due to turbulence suppression by the excitation. For small H (e.g. H/D=2), $Nu_{stag}$ is almost unchanged but the secondary peak much suppressed. On the other hand, $Nu_{stag}$ increases for H/D=6 due to turbulence enhancement by the excitation. The different behaviors of Nusselt number with respect to H/D are closely related to the changes in vortical structures by excitation.