  We point out that determination of the MNS matrix element |U_{e3}| = s_{13}
in long-baseline \nu_{\mu} \to \nu_e neutrino oscillation experiments suffers
from large intrinsic uncertainty due to the unknown CP violating phase \delta
and sign of \Delta m^2_{13}. We propose a new strategy for accurate
determination of $\theta_{13}$; tune the beam energy at the oscillation maximum
and do the measurement both in neutrino and antineutrino channels. We show that
it automatically resolves the problem of parameter ambiguities which involves
\delta, \theta_{13}, and the sign of \Delta m^2_{13}.
