We study curvature estimates of Ricci flow on complete 3-dim manifolds
without bounded curvature assumptions. Especially, from a more general
curvature preservation condition, we derived that nonnegative Ricci
curvature is preserved for any complete solution of 3-dim Ricci flow. A local
version of Hamilton-Ivey estimates is also obtained. Using that the nonnegative
Ricci is preserved under any 3-dim Ricci flow complete solution, we can prove the strong uniqueness of the Ricci flow with bounded nonnegative Ricci curvature and uniform injective radius lower bound as initial assumptions. This is joint work with Bing-Long Chen and Zhuhong Zhang.