  In \cite{CJ}, the authors show that the Cauchy problem of the Navier-Stokes
equations with damping $\alpha|u|^{\beta-1}u(\alpha>0,\;\beta\geq1)$ has global
weak solutions in $L^2(\R^3)$. In this paper, we prove the uniqueness, the
continuity in $L^2$ for $\beta>3$, also the large time decay is proved for
$\beta\geq\frac{10}3$. Fourier analysis and standard techniques are used.
