Natural 15N abundance method was used in this study to investigate the N2-fixing capacities of several herbaceous and shrub legumes by a field experiment. For herbaceous legumes, the results were in consistent with those obtained by a pot experiment. Crotalaria mucronata Desv. had a higher N2-fixing capacity than Vigna sinensis (L.) Savi. For shrub legumes, N2-fixing capacity under field condition was slightly different from that in pot experiment. These results demonstrated that the natural 15N abundance method was applicable to evaluating N2-fixing capacities of herbaceous and shrub legumes, and that pot experiment was suitable for evaluating the N2-fixing capacities of not only herbaceous legumes but also shrub legumes.Leguminous N2-fixing plants differed in response to phosphorus fertilization. Phosphorus fertilizer application greatly increased the percentage of nitrogen derived from air (% N dfa) and total amount of N2 fixed by Lespedeza formosa (Vog.) Koehne (Jiangxi). Phosphorus fertilizer showed no significant effect on the N2-fixing percentage of Cajanus cajan (L.) Millsp. but increased its total biomass, thus increasing the total amount of N2 fixed. Natural 15N abundance method was used in this study to investigate the the N2-fixing capacities of several herbaceous and shrub legumes by a field experiment. For herbaceous legumes, the results were in consistent with those obtained by a pot experiment. Crotalaria mucronata Desv. Had a The N2-fixing capacity than the Vigna sinensis (L.) Savi. For shrub legumes, N2-fixing capacity under field condition was slightly different from that in pot experiment. These results describe that the natural 15N abundance method was applicable to evaluating N2-fixing capacities of herbaceous and shrub legumes, and that pot experiment was suitable for evaluating the N2-fixing capacities of not only herbaceous legumes but also shrub legumes. Leguminous N2-fixing plants differed in response to phosphorus fertilization. Phosphorus fertilizer application greatly increased the percentage of nitrogen derived from air (% N dfa) and total amount of N2 fixed by Lespedeza formosa (Vog.) Koehne (Jiangxi). Phosphoru s fertilizer showed no significant effect on the N2-fixing percentage of Cajanus cajan (L.) Millsp. but increased its total biomass, thus increasing the total amount of N2 fixed.