We consider a priori estimates of Weyl’s embedding problem of (S^2,g) in general 3-dimensional Riemannian manifold (N^3,\bar g). We establish the mean curvature estimate under natural geometric assumption. Together with a recent work by Li-Wang, we obtain an isometric embedding of (S2,g) in Riemannian manifold. In addition, we reprove Weyl’s isometric embedding theorem in space form under the condition that g \in C^2 with D^2g Dini continuous.