  In this paper a general method to measure the longitudinal polarization of
$\tau$ lepton in the process $pp \rightarrow Z^{0} \rightarrow \tau\tau$ is
described. The method of optimal observable allows to distinguish between the
$\tau$ lepton helicity states with maximum possible sensitivity. The optimal
observable for the main $\tau$ decay modes ($e/\mu\nu\nu$, $\pi\nu$, $\rho\nu$,
$a_{1}\nu$) that can be identified by CMS or ATLAS detectors is described. The
analyzing power for $\tau$ polarization can be further gained by taking into
account a correlation between longitudinal spin states of $\tau^{+}$ and
$\tau^{-}$. The theoretical systematic uncertainty on the $\tau$ polarization
measured using the $\tau \rightarrow 3\pi\nu$ decays is estimated to be
$(\Delta P_{\tau})_{model} = (1.41 \pm 1.37)\times 10^{-4}$. This value is
approximately 50 times smaller than the best uncertainty on $\tau$ polarization
measurement given by combination of all LEP results. An approach to propagate
the measured $\tau$ polarization averaged over the $Z^{0}$ lineshape and parton
density functions to the electroweak parameters is also discussed.
