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The invasion of nonnative plants is considered one of the main threats to the structure and function of North American ecosystems. Moreover, they can alter ecosystem processes and reduce biodiversity. In arid and semi-arid region of North America, the species of European annual grass Bromus tectorum L. is an outstanding example of these problems, which not only increase the fire density and change the fire regime, but replace native communities. Therefore, there are amount of researches on B. tectorum, including resource acquisition, water use efficiency and growth. Whereas the relevant research on the morphology of diaspore is scare. Diaspores have a fundamental role in seed germination and seedling establishment. Besides, as an important link between different generations, diaspores have a vital significance on individual reproduction and population extension. Hence, diaspores under selection for studying have an important implication. This study compares differences in seed morphology for Bromus tectorum collected from the United States, Kazakhstan, and Xinjiang of China. The following indices of B. tectorum diaspores were analyzed: size, thickness of covering layers, and micromorphological characteristics of the base, middle and transition area of diaspores as well as of the awn. Micromorphology of the lemma and the cross-section of the diaspore were observed by scanning electron microscopy. Results showed that thickness of the lemma and the palea of diaspores from B. tectorum-infested grasslands in the United States were reduced (P < 0.05), likely because of environmental influences. This reduction facilitated the germination of diaspores and lowered the resistance of B. tectorum to adverse environmental conditions. The length of the awn also increased significantly (P < 0.05), which helped in dispersal and anchoring of diaspores. Therefore, B. tectorum adapted ecologically to its new environment in the United States by strengthening its establishment ability. However, the defense capability of B. tectorum decreased. These results fit the evolution of increased competitive ability hypothesis (EICA) of invasive species. Analysis of various cells on the lemma revealed that prickle densities and collapsed, long epidermal cells were easily influenced by environmental factors such as temperature and moisture because of the physiologic function of these structures on silicon accumulation. However, the form and the position of silica cells, which were not greatly influenced by environmental factors, might be genetically controlled. Studying these structures at the microscopic level helps define the relationship between the diaspore and its environment. This study has a reference value for future studies on B. tectorum.