Abstract: δ15N values of lettuce and soil were studied with different fertilizer conditions to prove the feasibility of using δ15N to discriminate between organic food and conventional food，4 treatments were laid out：Organic fertilizer in traditional broadcast way（N 400 kg ? hm-2），urea in partial fertilization way （N 300 kg ? hm-2），urea in traditional broadcast way（N 400 kg ? hm-2）and the control without fertilizer. The δ15N values of lettuce and soil in 4 treatment were significantly different（P control > urea in partial fertilization way > urea in traditional broadcast way. The temporal variations in the δ15N of lettuce and soils were similar among 4 treatments：Their δ15N values increased rapidly at first，then reduced slowly and stabilized finally. Maximum appeared in 20 days after transplanting. The δ15N value order of different lettuce organs is external part > root > internal part，and the difference is significantly（P < 0.05）. To sum up，the δ15N in inner of lettuce and soil can be used as a marker to reveal the history of nitrogen fertilization. It is useful tool for identifying the use of synthetic nitrogen fertilizer in the process of lettuce planting.

Key words: lettuce, organic food, isotopic fractionation, δ15N value