  We discuss the effects of isospin breaking which appear in the vertex
corrections for $Zb \bar{b}$, $Z\tau^+\tau^-$ and $W\nu\tau$ in a one-family
technicolor model without exact custodial symmetry. By means of the effective
lagrangian approach we compute the vertex corrections for $Zb\bar{b}$,
$Z\tau\tau$ and $W\tau\nu$ taking account of the contributions from
technivectormesons. If the isospin symmetry in technilepton sector is not
exact, technivectormesons contribute to the vertex correction for $Z\tau\tau$
but such contributions to the correction for $W\tau\nu$ are absent. If the
difference is measured, it is the evidence of the isospin breaking.
