  In this paper, employing a new inequality, we show that under certain
curvature pinching condition, the strictly convex closed smooth self-similar
solution of $\sigma_k^{\alpha}$-flow must be a round sphere. We also obtain a
similar result for the solutions of $F=-\langle X, e_{n+1}\rangle \, (*)$ with
a non-homogeneous function $F$. At last, we prove that if $F$ can be compared
with $\frac{(n-k+1)\sigma_{k-1}}{k\sigma_{k}}$, then a closed strictly
$k$-convex solution of $(*)$ must be a round sphere.
