In this note, we study the curvature flow to Nirenberg problem on $S^2$ with non-negative nonlinearity. This flow was introduced by Brendle and Struwe. Our result is that the Nirenberg problems has a solution provided the prescribed non-negative Gaus
sian curvature $f$ has its positive part, which possesses non-degenerate critical points such that $Delta_{S^2} f>0$ at the saddle points.
