A two-year small-plot field experiment was carried out to investigate the competitive relationship between seedlings of two important forage grasses, Leymus chinensis and Bromus inermis, under three levels of gap size (distance between seedlings and their neighbours) of 10, 20 and 40 cm in diameter, with neighbouring roots present or absent. Seeds of the two species were sown into artificially created gaps, in either monoculture or mixtures at equal rates. Results indicated that gap size had significant effects on seedling emergence, seedling survival and the growth performance of the two species, grown alone or in a mixture. Excluding the roots of neighbouring plants enhanced all the measured attributes and had significant effects on both the number of inflorescences and the biomass of B. inermis and on the biomass of L. chinensis. The performance of the two species differed significantly, both alone and in mixtures, with B. inermis outperforming L. chinensis consistently. The number of inflorescences of B. inermis was 2·6-fold higher than L. chinensis when grown in a mixture, and L. chinensis displayed a large and significant reduction in biomass when grown in a mixture with B. inermis (5·7 g plot−1) as compared with a monoculture (25·9 g plot−1). The relative yield (RY) for B. inermis exceeded 0·5, but for L. chinensis, it was below 0·5 in all treatments, and aggressivity (A) in all treatments was above 0. In addition, the relative competition intensity (RCI) of B. inermis was less than that of L. chinensis in all treatments, both separately and when averaged across treatments. We conclude that B. inermis is a superior competitor to L. chinensis in gaps, whereas the performance of L. chinensis was greatly suppressed in mixtures. This indicated that under similar conditions, B. inermis outcompetes L. chinensis during early growth stages. The results have practical implications for the restoration and revegetation of grasslands in northern China.