Seed size has been considered a key factor influencing the dynamics of plant communities, but few empirical, within-community predictions have been made based on seed size. Data on species density, biomass, frequency, and seed size were measured in alpine meadows with different grazing gradients in Northwest China. Seed mass variance and range within community were analyzed and relationships between seed size and density, biomass, and frequency within community were evaluated. Our results showed that vegetation height, coverage, and productivity decreased significantly with increasing grazing intensity. Species richness and total individual density were greatest in moderately-grazed meadow. Mean seed mass, mean coefficient of variation of seed mass, and seed mass range within community showed a consistent trend of moderately-grazed meadow > ungrazed meadow > heavily-grazed meadow. Correlation analyses showed that seed size was positively related to species density, biomass, and frequency within community in ungrazed meadow, but was negatively related to species density, biomass, and frequency within community in heavily-grazed meadow. Our study suggests that grazing disturbance can alter the relationship between seed size and dominance for coexistent species within communities. With increasing grazing intensity, the advantages of species density, biomass, and frequency were switched from the larger-seeded species to the smaller-seeded species within the community.