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In the narrow-line Seyfert 1 galaxy Mrk 766, a Quasi-Periodic Oscillation(QPO) signal with a period of $\sim6450$ s is detected in the \emph{XMM-Newton}data collected on 2005 May 31. This QPO signal is highly statisticalsignificant at the confidence level at $\sim5\sigma$ with the quality factor of$Q=f/\Delta f>13.6$. The X-ray intensity changed by a factor of 3 with rootmean square fractional variability of $14.3\%$. Furthermore, this QPO signalpresents in the data of all three EPIC detectors and two RGS cameras and itsfrequency follows the $f_{\rm QPO}$-$M_{\rm BH}$ relation spanning fromstellar-mass to supermassive black holes. Interestingly, a possible QPO signalwith a period of $\sim4200$ s had been reported in the literature. Thefrequency ratio of these two QPO signals is $\sim$ 3:2. Our result is also insupport of the hypothesis that the QPO signals can be just transient. Thespectral analysis reveals that the contribution of the soft excess componentbelow $\sim$ 1 keV is different between epochs with and without QPO, thisproperty as well as the former frequency-ratio are well detected in X-ray BHbinaries, which may have shed some lights on the physical origin of our event.

An X-ray periodicity of $\sim$1.8 hours in a narrow-line Seyfert 1 galaxy Mrk 766