Ageratina adenophora is a noxious invasive weed worldwide. However, the mechanisms underlying its invasiveness are still not well elucidated. To explore the effects of seed characteristics on invasiveness of this invader, A. adenophora was compared with three native species in terms of seeds morphological traits and germination responses to environmental factors including temperatures, water, and salt. The invader was significantly lower in seed weight, length, width, and pappus length than three phylogenetically closely related native plant species. Seed settlement velocity was lower for the invader although the difference between the invader and two natives (E. japonicum and E. chinense) was not significant. Small seeds facilitated spread of the invader. Compared with the seeds of two natives (E. japonicum and E. chinense), seeds of the invader germinated in a wider range of temperature. In sublow temperature (15 and 20 ℃), germination ratio and germination index were higher for the invader than for two natives (E. japonicum and E. stoechadosmum). Responses of seed germination to water and salt stress were stronger for the invader relative to three natives. At suitable conditions (without water and salt stress), however, germination ratio and germination index were higher for the invader than for three natives. These traits mentioned above may facilitate successful invasion of A. adenophora.