An r-uniform graph G is dense if and only if every proper subgraph G ′ of G satisfies λ(G ′ ) < λ(G), where λ(G) is the Lagrangian of a hypergraph G. In 1980's, Sidorenko showed that π(F ), the Turán density of an r-uniform hypergraph F is r! multiplying the supremum of the Lagrangians of all dense F -hom-free r-uniform hypergraphs. This connection has been applied in estimating Turán density of hypergraphs. When r = 2, the result of Motzkin and Straus shows that a graph is dense if and only if it is a complete graph. However, when r ≥ 3, it becomes much harder to estimate the Lagrangians of r-uniform hypergraphs and to characterize the structure of all dense r-uniform graphs. The main goal of this note is to give some sufficient conditions for 3-uniform graphs with given substructures to be dense. For example, if G is a 3-graph with vertex set [t] and m edges containing [t − 1] (3) , then G is dense if and only if m ≥ t−1 3 + t−2 2 + 1. We also give sufficient condition condition on the number of edges for a 3-uniform hypergraph containing a large clique minus 1 or 2 edges to be dense.