A note on compact K\"ahler-Ricci flow with positive bisectional curvature

We show that for any solution to the K"ahler-Ricci flow with positive bisectional curvature on a compact K"ahler manifold $M^n$, the bisectional curvature has a uniform positive lower bound. As a consequence, the solution converges exponentially fast to an K"ahler-Einstein metric with positive bisectional curvature as t tends to the infinity, provided we assume the Futaki-invariant of $M^n$ is zero. This improves a result of D. Phong, J. Song, J. Sturm and B. Weinkove in which they assumed the stronger condition that Mabuchi K-energy is bounded from below.