Elevated atmospheric CO2 concentration ([CO2]) stimulates seed mass production in many species, but the extent of stimulation shows large variation among species. We examined (1) whether seed production is enhanced more in species with lower seed nitrogen concentrations, and (2) whether seed production is enhanced by elevated [CO2] when the plant uses more N for seed production. We grew 11 annuals in open top chambers that have different [CO2] conditions (ambient: 370 μmol mol−1, elevated: 700 μmol mol−1). Elevated [CO2] significantly increased seed production in six out of 11 species with a large interspecific variation (0.84–2.12, elevated/ambient [CO2]). Seed nitrogen concentration was not correlated with the enhancement of seed production by elevated [CO2]. The enhancement of seed production was strongly correlated with the enhancement of seed nitrogen per plant caused by increased N acquisition during the reproductive period. In particular, legume species tended to acquire more N and produced more seeds at elevated [CO2] than non-nitrogen fixing species. Elevated [CO2] little affected seed [N] in all species. We conclude that seed production is limited primarily by nitrogen availability and will be enhanced by elevated [CO2] only when the plant is able to increase nitrogen acquisition.