  We theoretically study the orbital destructive effect against
superconductivity in a parallel magnetic field in the
Fulde-Ferrell-Larkin-Ovchinnikov (FFLO or LOFF) phase at zero temperature in a
quasi-two-dimensional (Q2D) conductor. We demonstrate that at zero temperature
a special parameter, $\lambda = l_{\perp}(H)/d$, is responsible for strength of
the orbital effect, where $l_{\perp}(H)$ is a typical "size" of the
quasi-classical electron orbit in a magnetic field and $d$ is the inter-plane
distance. We discuss applications of our results to the existing experiments on
the FFLO phase in the organic Q2D conductors $\kappa$-(ET)$_2$Cu(NCS)$_2$ and
$\kappa$-(ET)$_2$Cu[N(CN)$_2$]Cl.
