Abstract: Planar laser-induced fluorescence (PLIF) is one of the most important and widely-used laser diagnostic techniques in combustion studies. But it is still a challenge to apply OH-PLIF measurements in high-pressure combustion, such as practical engines. In order to study the effects of pressure on OH-PLIF, this paper presented OH-PLIF measurements of CH₄/air flat flames at high pressures. An OH-PLIF measurement set-up was established, and the measurements were carried out on a newly developed high-pressure burner. The fluorescence yields were numerically simulated under different combustion conditions in the experiment. The measurement results were analyzed and compared with the numerical simulation of fluorescence yield. It is found that the fluorescence decreases sharply as pressure increases. The peaks of fluorescence become closer to the burner exit at high pressures, because the thickness of flat flames becomes thinner when the pressure increases. According to the comparison between the experiment and simulation results, the fluorescence quenching effect is dominated in the pressure dependence of fluorescence.