  In this paper, we first derive a pinching estimate on the traceless Ricci
curvature in term of scalar curvature and Weyl tensor under the Ricci flow.
Then we apply this estimate to study finite-time singularity behavior. We show
that if the scalar curvature is uniformly bounded, then the Weyl tensor has to
blow up, as a consequence, the corresponding singularity model must be Ricci
flat with non-vanishing Weyl tensor.
