The mechanisms underlying biological invasions are still not well elucidated. In this study, ecophysiological traits of invasive Eupatorium adenophorum and native E. japonicum were compared at 10 irradiances in field. I hypothesized that the invader may allocate a higher fraction of leaf nitrogen (N) to photosynthesis and have higher light-saturated photosynthetic rate (Pmax) and specific leaf area (SLA) than E. japonicum. The invader had a significantly higher ability to acclimate to high irradiance than E. japonicum, while it showed a similar shade-tolerant ability. The invader indeed allocated a higher fraction of leaf N to photosynthesis than E. japonicum, which, with its high leaf N content (NA), resulted in a higher N content in photosynthesis (NP), contributing to its higher biochemical capacity for photosynthesis and Pmax. However, the invader had a significantly lower SLA than E. japonicum, contributing to its higher Pmax but increasing its area-based leaf construction cost. The abilities to acclimate to a wider range of irradiance and to allocate a higher fraction of leaf N to photosynthesis, and the higher Pmax, NA, NP and leaf area ratio may contribute to the invasion of the invader. High SLA is not always necessary for invasive species.