Recent studies of active galactic nuclei (AGN) found a statistical inverse linear scaling between the X-ray normalized excess variance $sigma_{rm rms}^2$ (variability amplitude) and the black hole mass spanning over $M_{rm BH}=10^6- 10^9 M_{odot}$. Being suggested to have a small scatter, this scaling relation may provide a novel method to estimate the black hole mass of AGN. However, a question arises as to whether this relation can be extended to the low-mass regime below $sim10^6 M_{odot}$. If confirmed, it would provide an efficient tool to search for AGN with low-mass black holes using X-ray variability. This paper presents a study of the X-ray excess variances for a sample of AGN with black hole masses in the range of $10^5- 10^6 M_{odot}$ observed with {it XMM-Newton} and {it ROSAT}, including data both from the archives and from newly preformed observations. It is found that the relation is no longer a simple extrapolation of the linear scaling; instead, the relation starts to flatten at $sim10^6 M_{odot}$ toward lower masses. Our result is consistent with the recent finding of citet{L15}. Such a flattening of the $M_{rm BH}-sigma_{rm rms}^2$ relation is actually expected from the shape of the power spectrum density of AGN, whose break frequency is inversely scaled with the mass of black holes.
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