The effect of mainstream oscillations on spatio-temporal variation of adiabatic effectiveness was quantified extensively by the fast-response pressure-sensitive paint (fast-PSP) technique. During the experiment, nitrogen (N-2) was used as the coolant and was discharged from a 35 degrees-inclined round hole. While the steady state (i.e., f = 0 Hz) was selected as the baseline, the oscillating cases were assessed in great detail to explore the frequency effect (i.e.,f = 5, 10, and 20 Hz with a fixed amplitude) on film cooling performance. Measured instantaneous effectiveness results (i.e., eta) were analyzed in terms of the ensemble averaged (i.e., (eta) over bar) and phase-averaged results (i.e., (eta) over tilde), where the latter one was further analyzed through triple decomposition. The ensemble-averaged results showed a strong effect of mainstream oscillations on adiabatic effectiveness: compared to the steady state, a reduction of 30-35% was observed in effectiveness for the M = 0.4 at f = 20 Hz cases. As for the phase-averaged results, a relatively weak dependence was observed at 5 Hz with respect to the phase angles. However, as the frequency increased to 20 Hz, obtained effectiveness exhibited a strong correlation with the velocity phase, where its contour was found to become longer and shorter during periodic cycles. In general, mainstream oscillations have a significant impact on the film cooling behavior. They not only degrade the cooling effectiveness, but also increase the unsteadiness in film cooling. (C) 2018 Elsevier Ltd. All rights reserved.