Let G be a graph, and let n, k and d be three nonnegative integers such that n+2k+d≤ |V(G)|-2 and, |V(G)| and n+d have the same parity. If after deleting any n vertices from G the remaining subgraph of G contains an k-matching and each k-matching of the subgraph can be extended to a defect-d-matching of the subgraph, then G is called an (n, k, d)-graph. Liu and Yu[1] first introduced (n, k, d)-graphs, and gave some properties and characterization of (n, k, d)-graphs. A (0, k, 1)-graph may be also called a near k-extendable graph. In the present paper, the authors improve the characterization of (n, k, d)-graphs, and consequently obtain a characterization of near k-extendable graphs. Furthermore, a characterization of near k-extendable bipartite graphs and the relations between near k-extendable graphs and n-factor critical graphs are investigated.