A method and an apparatus for identifying sub-graphs similar to a sample sub-graph from a network, in particular for identifying similar community structures from a network, are disclosed. Specifically, when a sample sub-graph is a community structure, it usually contains a plurality of nodes densely associated with each other. The method proposes first screening the network via key nodes of community structures so as to find sub-graph candidates that are potentially similar to the sample sub-graph, then strictly comparing the sub-graph candidates with the sample sub-graph to find similar sub-graphs of the sample sub-graph.