  We prove that any $n$-dimensional closed mean convex $\lambda$-hypersurface
is convex if $\lambda\le 0.$ This generalizes Guang's work on $2$-dimensional
strictly mean convex $\lambda$-hypersurfaces. As a corollary, we obtain a gap
theorem for closed $\lambda$-hypersurfaces with $\lambda\le 0.$
