An extensive root system is one of the highly desirable traits for plant growth and adaptation to environmental stresses which is affected by the availability of atmosphere CO 2 and nitrogen (N) nutrition. The objectives of this study were to examine the interactive effects of elevated CO 2 and nitrogen on root growth and underlying regulatory mechanisms related to hormones metabolism in tall fescue ( Festuca arundinacea ). Plants (cv. ‘Barlexas’) were hydroponically grown in nutrient solution with low N (0.25 mM) and moderate N (4 mM) conditions and exposed to ambient CO 2 concentration (400 µmol·mol −1 ) or elevated CO 2 concentration (800 µmol·mol −1 ) for 35 days. Elevated CO 2 differentially promoted root growth under different nitrate levels. The longest root length, root biomass and all types of roots including the length, number, volume and surface area of both crown and lateral roots were enhanced by elevated CO 2 under moderate nitrate but without significant effects on crown root number under low nitrate condition. The extent of elevated CO 2 -promotion on lateral root was greater under moderate nitrate than that under low nitrate condition. The improvement of elevated CO 2 on root growth under moderate nitrate condition was related with increased content of indoleacetic acid (IAA) and isopentenyl adenosine (iPA) through up-regulating genes in auxin synthesis, cytokinin synthesis and down-regulating cytokinin degradation gene. Our results indicated that elevated CO 2 had differential promotive effects on crown and lateral root growth in tall fescue under low and moderate nitrate conditions in association with the regulation of endogenous auxin and cytokinin metabolism. • Elevated CO 2 had differentially positive effects on fibrous root characteristics under different nitrate supply. • Elevated CO 2 -regulated root growth under moderate nitrate was related with the endogenous auxin and cytokinin metabolism. • Elevated CO 2 -induced the higher IAA content was mainly attributed to the up-regulation of IAA synthesis gene. • Elevate CO 2 caused iPA accumulation by the up-regulation of synthesis gene and the down-regulation of degradation gene.