In this presentation, we talk about the stability of the Willmore functional. For an integral 2-varifold in  with square integrable generalized mean curvature and finite mass, if its Willmore energy is smaller than  and the mass is normalized to be , we show that  is  and bi-Lipschitz close to the standard embedding of the round sphere in a quantitative way when  For , we prove that the sharp constant is  This is a joint work with Dr. Yuchen Bi at Peking University.