Abstract: Let $G$ be a simple  graph containing a perfect matching.  $G$ is said to be bipartite matching extendable (BM-extendable) if every matching $M$ whose induced subgraph is a bipartite graph extends to a perfect matching. Extremal graph problems are at the core  of graph theory. In this paper, we characterize maximally BM-unextendable graphs, maximally BM-extendable graphs in the class of complete $k$-partite graphs with $k\geq 2$.