  The models of large extra compact dimensions, as suggested by Arkani-Hamed,
Dimopoulos and Dvali, predict exciting phenomenological consequences with
gravitational interactions becoming strong at the TeV scale. Such theories can
be tested at the existing and future colliders. In this paper, we study the
contribution of virtual Kaluza-Klein excitations in the process $e^+e^- \to t
\bar t H $ at future linear collider (NLC). We find that the virtual exchange
KK gravitons can modify the cross-section $\sigma(e^+e^- \to t \bar t H)$
significantly from its Standard Model value and will allow the effective string
scale to be probed up to 7.9 TeV.
