  We study the flavor universality of the lepton couplings with the W and Z
bosons. Currently, it is constrained by the electroweak precision observables.
We focus on the leptonic decay of the pion. It is shown that the decay is
expected to have a comparable or better sensitivity to the lepton-flavor
universality in the near future. Furthermore, it provides a complementary
information in determining the Wlnu couplings without assuming new physics
models. This result is applied to a model of the extra lepton.
