  We calculate the ${\cal O}(\alpha_{{\rm ew}})$ electroweak radiative
corrections to $e^+ e^- \to t \bar{t} h$ at a electron-positron linear collider
(LC) in the standard model. We analyze the dependence of the ${\cal
O}(\alpha_{{\rm ew}})$ corrections on the Higgs boson mass $m_{h}$ and
colliding energy $\sqrt{s}$, and find that the corrections significantly
decrease or increase the Born cross section depending on the colliding energy.
The numerical results show that the ${\cal O}(\alpha_{{\rm ew}})$ relative
correction is strongly related to the Higgs boson mass when $\sqrt{s}=500 GeV$,
and for $m_h = 150 GeV$ the relative correction ranges from -31.3% to 2.3% as
the increment of the colliding energy from 500 GeV to 2 TeV.
