  Using the Monte Carlo method, we determine the free energy of the interface
of the 3D Ising model in the scaling region. By integrating the interface
energies over the inverse temperature $\beta$, we obtain estimates for the free
energies of interfaces with cross sections up to 96 by 96, and for a range
$0.223 \leq \beta \leq 0.23$. Our data yield a precise estimation of the
interface tensions $\sigma$. We determine the amplitude $\sigma_0$ in the
critical law $\sigma \sim \sigma_0 t^{\mu}$ and estimate the combination
$\sigma \xi^2$ which yields the universal constant $R_{-}$ in the critical
limit.
