The unsaturation degree of cellular membrane lipids is mainly determined by fatty acid desaturases (FADs), and both the unsaturation degree and FADs are closely related to the tolerance of plants to harsh environments such as low temperature and salt stress. Ammopiptanthus mongolicus is the only evergreen xerophytic broadleaf plant that is endemic to the central Asian desert and has very strong tolerance to abiotic stresses such as cold, drought and high salinity. In this study, both the cDNA and gDNA fragments for an oleic acid desaturase, AmFAD21, in this species were cloned by RTPCR, and the gDNA region has no intron. Except for the conserved histidine clusters for FAD family, there also exist the signal peptide for endoplasmic reticulum localization and transmembrane domains in the AmFAD21 sequence (containing 382 amino acid residues). Phylogenetic relationship analysis showed that AmFAD21 is closely related to the FAD2s from leguminous plants such as Glycine max and Caragana korshinskii but is distantly related to the FAD2s from other plant species such as Zea mays and Arabidopsis thaliana. qRTPCR analysis showed that in laboratorycultured A. mongolicus seedlings, the expression level of AmFAD21 increased significantly between 2 to 24 h of the cold treatment and between 6 to 13 d of the drought treatment. Moreover, AmFAD21 showed an expression pattern of increasing, decreasing and increasing again during the period of salt treatment. In young leaves of the A. mongolicus plants naturally growing in the wild, the expression levels of AmFAD21 during the autumn and winter months were overall obviously higher than those in the spring and summer. The plant expression vector p330035SAmFAD21 was successfully constructed and wild type Arabidopsis was transformed using the Agrobacterium tumefaciensmediated method. In total, 16 AmFAD21 transgenic plants were obtained. Stress tolerance evaluation experiments revealed that the transgenic lines had significantly higher freezing tolerance compared with the wild type Arabidopsis; their drought and salt tolerances were also improved.