  The deviation between the standard model prediction and the measurement of
the muon g-2 is currently at 3-4 sigma (can be up to 7 sigma in the upcoming
experiment E989). If new physics is responsible for such discrepancy, it is
expected that the new contributions to tau g-2 are even larger than that for
muon due to its large mass. In spite of that, the tau g-2 is much more
difficult to be directly measured because of its short lifetime. In this paper,
we consider the effect of the tau g-2 at $e^-e^+$ colliders in a model
independent way. Using the tau pair production channel at the Large Electron
Position Collider (LEP), we have found the allowed range for the new physics
contribution of the tau g-2 assuming a $q^2$-dependence ansatz for the magnetic
form factor. In our analysis, we take into account the standard model one-loop
correction as well as the initial state photon radiation. We have also
investigated the prospect at future $e^-e^+$ colliders, and determine the
expected allowed range for the new physics contribution to the tau g-2. Given
the proposed beam polarization configuration at the International Linear
Collider (ILC), we have analyzed the dependence of this allowed range on the
integrated luminosity as well as the relative systematic error.
