The aim of this study is to propose a predictive grouping maintenance strategy for complex structure systems with gradually deteriorating units. The unit's conditions are inspected at regular time intervals. Preventive maintenance decision rule is mainly based on the group improvement importance measure defined as a ratio of the improvement ability on the system reliability of a group of several units divided by the total maintenance costs of the group. Economic dependencies as well as structure dependencies between units are also investigated. A numerical example of a complex structure of 11-unit system is finally introduced to illustrate the uses and the advantages of the proposed maintenance strategy.