  In the framework of the large extra dimensions (LED) model, the effects of
LED on the processes \rrtth and \eetth at future linear colliders are
investigated in both polarized and unpolarized collision modes. The results
show that the virtual Kaluza-Klein (KK) graviton exchange can significantly
modify the standard model expectations for these processes with certain
polarizations of initial states. The process \rrtth with $\sqrt{s}=3.5 TeV$
allows the effective scale $\Lambda_T$ to be probed up to 7.8 and 8.6 TeV in
the unpolarized and $P_{\gamma} = 0.9$, J=2 polarized $\gamma \gamma$ collision
modes, respectively. For the \eetth process with $\sqrt{s}=3.5 TeV$, the upper
limits of $\Lambda_T$ to be observed can be 6.7 and 7.0 TeV in the unpolarized
and $P_{e^+} = 0.6$, $P_{e^-} = 0.8$, $-+$ polarized $e^+e^-$ collision modes,
respectively. We find the \rrtth channel in J=2 polarized photon collision mode
provides a possibility to improve the sensitivity to the graviton tower
exchange.
