Narrowleaf vetch (Vicia angustifolia L.) (NLV) is a legume widely distributed in the Eurasian continent. Although commonly growing as a weed in cultivated areas, NLV could become a suitable species for cultivation as a green manure crop to increase the nitrogen content of soil. We examined NLV and a common green manure crop, Chinese milk vetch (Astragalus sinicus L.) (CMV), both in monocultures and in mixed cropping of the 2 species. The nitrogen-fixing activity and root nodule activity of the species were determined and the effects on the nitrogen (N) supply to soil were evaluated. CMV produced more plant N than NLV, largely as a response to better growth after the over-wintering period. The amount of atmospheric N fixed by NLV (4.3 g m^-2) was only half of that fixed by CMV (8.6 g m^-2). Nevertheless, the amount of N in NLV (up to 14 g m^-2) was sufficient to supply the demand of the following crop. The mixed culture of CMV and NLV performed better than the monoculture. Since developing countries cannot afford to invest money on nitrogenous fertilizers or legume seeds, the use of the widely distributed NLV as a green manure might be effective and contribute to the sustainability of cropping systems.