  Hypervelocity stars (HVSs) ejected by the massive black hole at the Galactic
center have unique kinematic properties compared to other halo stars. Their
trajectories will deviate from being exactly radial because of the asymmetry of
the Milky Way potential produced by the flattened disk and the triaxial dark
matter halo, causing a change of angular momentum that can be much larger than
the initial small value at injection. We study the kinematics of HVSs and
propose an estimator of dark halo triaxiality that is determined only by
instantaneous position and velocity vectors of HVSs at large Galactocentric
distances (r>~50kpc). We show that, in the case of a substantially triaxial
halo, the distribution of deflection angles (the angle between the stellar
position and velocity vector) for HVSs on bound orbits is spread uniformly over
the range 10--180deg. Future astrometric and deep wide-field surveys should
measure the positions and velocities of a significant number of HVSs, and
provide useful constraints on the shape of the Galactic dark matter halo.
