  Kink instability is a possible mechanism for solar filament eruption.
However, the twist of a solar filament is very difficult to directly measure
from observation. In this paper, we carried out the measurement of the twist of
a solar filament by analyzing its leg rotation. An inverse S-shaped filament in
active region NOAA 11485 was observed by the Atmospheric Imaging Assembly (AIA)
of Solar Dynamics Observatory (SDO) on 2012 May 22. During its eruption, the
leg of the filament exhibited a significant rotation motion. The 304 \AA\
images were used to uncurl along the circles, whose centers are the axis of the
filament's leg. The result shows that the leg of the filament rotated up to
about 510 degrees (about 2.83$\pi$) around the axis of the filament within
twenty-three minutes. The maximal rotation speed reached 100 degrees per minute
(about 379.9 km/s at radius 18$^\prime$$^\prime$), which is the fastest
rotation speed that has been reported. We also calculated the decay index along
the polarity inversion line in this active region and found that the decline of
the overlying field with height is not so fast enough to trigger the torus
instability. According to the condition of kink instability, it is indicating
that the kink instability is the trigger mechanism for the solar filament
eruption.
