Reliability importance measures are widely used for decision-aiding in reliability studies, risk analyses and maintenance optimization. We propose a novel time-dependent importance measure for multi-non repairable component systems. The proposed importance measure of a component/group of components is defined as its ability in improving the system reliability given the components' condition. To take into account economic aspects (e.g., maintenance costs, economic dependence between components and the cost benefit thanks to maintenance operations), an extension of the proposed importance measure is then investigated. Thanks to these proposed importance measures, the component/group of components can be "optimally" selected for preventive maintenance regarding to the reliability criteria and/or the financial issues. A numerical example of a 5-component system is introduced to illustrate the use and the advantages of the proposed importance measures.