In this paper, we propose a novel importance measure, namely LIM, which is defined as the improvement ability in the system residual life when replacing a component/group of components at a given time. LIM measure allows considering the current condition (state or degradation level) of all components at given time and the system structure into a single metric to rank a component/group of components regarding to the system life time improvement ability. Moreover, to take into account economic aspects (e.g., preventive maintenance costs, benefit gained by preventive maintenance and economic dependence between components), an extension of LIM measure is then investigated. Thanks to LIM measure and its extension, a component/group of components can be ”optimally” selected for preventive maintenance regarding to the technical criterion (residual life of the system) and/or the economic issues (benefit and cost). A numerical example of a 4-component system is introduced to illustrate the use and the advantages of the proposed importance measures.