Abstract: A pot experiment was carried out to study the nitrate content difference among three lettuce varieties and the effects of application of nitrogen fertilizer on nitrate accumulation.The result of this experiment indicated that the nitrate content of three varieties of lettuce is obviously different at the same N rate and there were significant differences in nitrate content at different N rates,and so does NRase activities . Furthemore ,nitrate content and NRA go up horizontally with the N supply levels .Therefore ,both genotype and N supply levels should be taken into account carefully in vegetable production to reduce nitrate concentration in vegetable.