  We have investigated the effect of thermal fluctuation on the interlayer
coupling of three-dimensional Josephson-junction arrays with anisotropic
interactions as a model of layered high-$T_c$ oxides, focusing on non-Ohmic
current-voltage characteristics. Langevin dynamic simulations were performed
for various Josephson-coupling anisotropies and for various temperatures in
both zero and finite magnetic fields. We find that, in the highly anisotropic
region, the interlayer coupling, which causes the non-vanishing critical
current ($I_{c}$), is suppressed by thermal fluctuations, and tends to push up
the Kosterlitz-Thouless transition temperature ($T_{KT}$) of the
two-dimensional (2D) regime. In this highly anisotropic region, $I_c$ begins to
decrease drastically near $T_{KT}$. This suggests that interlayer coupling is
screened by thermally excited 2D small vortex-antivortex pairs as well as the
intralayer logarithmic part of vortex-antivortex interactions. Moreover, weak
magnetic fields parallel to the $c$-axis affect only the
logarithmic-interaction part between the 2D pancake vortices.
