Effects of nitrogen incorporated with 15N-nitrogen, which was applied to grown alone and in mixture with grass and legumes of forage crops, on the yield of dry matter, total nitrogen content, percentage of nitrogen derived from fertilizer (P.N.D.F.), A-value were studied, and the results obtained are as follows: 1. With nitrogen for the yield of dry matter was highly effective only to orchardgrass and were also effect to the early stages of alfalfa and birdsfoot trefoil in case of a mixed sowing. 2. Alone and mixed sowings showed the yield of dry matter in decreasing order of alfalfa<red clover>orchardgrass>birdsfoot trefoil, and orchardgrass + alfalfa>orchardgrass + red clover>orchardgrass + birdsfoot trefoil, respectively. 3. In case of grown alone, orchardgrass gave higher percentage of nitrogen derived from fertilizer (P.N.D.F.) than legume, whereas in case of grown in mixture it was in reverse order throughout the plots. It appears likely that orchardgrass was rather supplied soil nitrogen by legume then by nitrogen application. And P.N.D.F. was getting smaller in the latter stage of all forage crops in case of grown alone. In case of grown in mixture, however, and inconsistent P.N.D.F. was obtained from orchardgrass. 4. The A-value in case of grown alone was decreased in order of alfalfa>red clover>orchardgrass>birdsfoot trefoil. In contrast, however, the A-value in case of grown in mixture wes decreased in order of orchardgasss+alfalfa (3:7)>orchardgrass+red clover (3:7)>orchardgrass+red clover (3:7)>orchardgrass+alfalfa(5:5)>orchardgrass+birdsfoot trefoil (5:5)>orchardgrass+red clover (5:5)>orchardgrass+birdsfoot trefoil (3:7).