After enterprise systems (ES) are infused to daily businesses, employees’ extended use has already become an important concern to firms striving for reaping benefits from IT investment. Considering the innovative and extra-role nature of extended use, the paper further re-conceptualizes extended use as proactive behaviour. And based on the theory of proactive motivation, a research model is developed to explain how three specific antecedents, i.e., system self-efficacy, leader-member exchange, and system modularity jointly impact employees’ extended use. The model is tested with a survey of enterprise system users in six firms which have already implemented ERP systems, and several meaningful findings are yielded. First, except for leader-member exchange, both system self-efficacy and modularity can positively and directly affect extended use. Second, leader-member exchange, rather than exerting a direct effect, can positively moderate the effects of system self-efficacy and modularity on extended use. Third, system modularity can strengthen the relationship between system self-efficacy and employees’ extended use. The limitations and implications for research and practical fields are discussed.