  The photometric mass ratios of eclipsing binaries are usually estimated by
light-curve modeling with an iterative method. We propose a new method for
estimating the photometric mass ratio of an overcontact binary using the
derivatives of a light curve, which provides a reasonable uncertainty value.
The method mainly requires only the time interval value between two local
extrema found in the third derivative of a light curve, with no need of an
iterative procedure. We applied the method to a sample of real overcontact
binary data and compared the estimated mass ratios with their spectroscopically
determined values. The comparison showed that our estimated mass ratios for
$\sim $67% of the samples agreed with their spectroscopic mass ratios within
the estimated uncertainties, and the errors for 95% of them are within $\pm
0.1$. Our method should be useful for estimating mass ratios for numerous
overcontact eclipsing binaries found with existing and future surveys, as well
as for the light-curve analysis of each system.
