This　paper　is　engaged　in　identifying　weak　substructures　of　a　vibration　system　and　modifying　the　parameters　of　the　system　using　analysis　technique　of　unit　volume　energy.　For　any　main　vibration　mode　of　a　system,　defining　the　substructures　with　large　unit　volume　energy　as　weak　substructures,　the　modification　method　of　physical　parameters　of　the　system　is　presented.　Along　with　it,　the　analysis　method　of　unit　volume　energy　for　virtual　substructures　is　presented,　which　is　used　for　structure　configuration　modification.　The　analysis　technique　presented　in　this　paper　is　used　directly　to　modify　physical　parameters　of　the　weak　substructures.　The　practical　application　of　the　method　shows　that　it　is　simple　and　efficient,　which　avoids　complex　sensitivity　analysis　in　common　methods　of　structure　modification　or　optimization　design.　(C)　2003　Elsevier　B.V.　All　rights　reserved.