Hairy vetch (Vicia villosa Roth), leguminous green manure crop, can increase soil fertility and reduce chemical nitrogen application for cash crop such as spring corn. More yield of hairy vetch is needed at planting cash crop to obtain higher effect of green manure. Hairy vetch was seeded on Sep. 10, Oct. l, and Oct. 20 and at 10, 20, 30 and 40 ㎏/ha of seeding rate respectively, in 1996 and 1997. Dry matter and nitrogen yield of hairy vetch were measured on May 1 in 1997 and 1998. Proper seeding rate of hairy vetch was 30 ㎏/ha irrespective of years and seeding dates. Above-ground dry matters of hairy vetch on May 1 in 1997 and 1998 decreased according to delayed seeding, and those were 5.5∼7, 4∼4.5, 1.3∼2.2 ton/ha on Sep. 10, Oct. 1 and Oct 20 of seeding date, respectively in seeding rate 30∼40 ㎏/ha. Also nitrogen yield of hairy vetch on May 1 decreased according to delayed seeding, and those were 220∼280, 160∼180, 60∼100 ㎏/ha on Sep. 10, Oct. l and Oct 20 of seeding date, respectively in seeding rate 30∼40 ㎏/ha. Therefore, we suggest that hairy vetch has to be seeded earlier in autumn to obtain high green manure yield in spring. To determine the detailed optimum seeding time in autumn, hairy vetch was seeded on Aug. 20, Aug. 31, Sep.10, Sep. 20, and Sep. 30 in 1999 and was harvested on April 22, April 27, and May 2 in 2000, respectively. Dry matter and nitrogen yield of hairy vetch by seeding in late August were higher than those by seeding in September indicating that dry matter of hairy vetch were 7∼8, 6∼7, 4∼5, 2∼3 ton/ha and nitrogen yield were 240∼290, 200∼260, 150∼220, 70∼120 ㎏/ha, respectively when seeded on Aug. 20, Aug. 31, Sep. 10, Sep. 20 and Sep. 30 and harvested on April 22∼May 2. Increase of dry matter and N yield of hairy vetch by 10 days delayed harvest was higher in late August seeding than in September seeding. So hairy vetch should be seeded in late August if possible to obtain much more green manure yield and be seeded until September because green manure yield decrease rapidly when seeded after October.