Subassembly approach has been applied by today's industry to reduce the complexity of assembly sequencing for complex assemblies. By analyzing assembly mating conditions, a connection-contact graph is constructed to identify the key components for subassembly extraction. After removing the connection between key components, independent and stable subassemblies can be obtained. In addition, assembly time is applied to evaluate different subassembly combinations for the selection of better subassembly extraction. Matrix representation and algorithm are proposed for computerization. Finally, an example is presented for the study.