Abstract: The theory of ecomorphology predicts that species with similar morphological traits can occupy similar ecological niche, which may cause competitive exclusion. To apply this theory into fish invasion ecology research is of significance for understanding the interaction between native and invasive species. Here, we compared the morphological difference between two native (Oryzias pectoralis, Oryzias curvinotus) and one invasive species (Gambusia affinis) to explore the competitive exclusion among them. The results showed that despite O. pectoralis and O. curvinotus were sympa-tric species, they varied in spatial distribution. Such a result supported the theory of ecomorphology, which predicts that two species with similar morphological traits might have strong competition. Moreover, their population density exhibited a significant negative relationship with that of G. affinis. The morphology of G. affinis and both Oryzias species were more similar when comparing to other fish in the assemblage. Results from the cluster analysis showed that G. affinis and Oryzias species were close in a branch, with extremely low spatial niche overlap between invasive mosquitofish and native Oryzias species. There was significant negative correlation between the population abundance of mosquitofish and Oryzias species. All the results suggested that mosquitofish led to population decline of both Oryzias species, due to the ecomorphological similarity. More studies are needed to better understand the mechanisms of G. affinis invasion in habitats of native Oryzias species.