The functions of two fatty acid desaturase genes, AmFAD7 and AmFAD8, from Ammopiptanthus mongolicus with very strong tolerance to abiotic stresses were analyzed. The results showed that: (1) AmFAD7 and AmFAD8 gene encode 455 and 457 amino acids, respectively. (2) The semiquantitative RTPCR results showed that for young leaves of the A. mongolicus plants growing in the wild, the expression level of AmFAD7 was very low in summer and quite high in winter, and it was slightly lower in both spring and autumn than that in winter. The expression levels of AmFAD8 in the leaves were higher in spring, autumn, and early winter than those in other months. The expression level of AmFAD7 reduced firstly and then increased obviously, while the expression of AmFAD8 increased slightly compared with that before the treatment in the A.mongolicus seedlings under cold (4 ℃ to -6 ℃ gradient cooling) treatment for 2 to 48 h. Under heat (42 ℃) treatment for 2 to 48 h, the expression of both genes was inhibited, and the suppression of AmFAD7 was particularly rapid and marked. During dehydration (25 ℃) treatment for 2 to 48 h, the expression of AmFAD7 showed an obvious increase, while that of AmFAD8 had a slight decrease. (3) The constructions of plant expression vectors p330035SAmFAD7 and p330035SAmFAD8 of AmFAD7 and AmFAD8 genes were introduced into Arabidopsis to obtain eighteen and sixteen independent transgenic lines, respectively. After being examined by semiquantitative RTPCR, two (F71 and F72) and four (F81 to F84) transgenic lines showed high transcription level of AmFAD7 and AmFAD8, respectively. (4) The relative electrolytic leakage (REL) analysis showed that at normal condition (22 ℃), the REL of AmFAD7 transgenic lines and wild type had no significant difference. After treatment at -1 ℃ for 24 h, the REL levels in F71 and F72 (32.8 % and 36.1%, respectively) were significantly lower than that of wild type (48.5%). And after heat treatment at 37 ℃ for 24 h, the REL levels in F71 and F72 (44.7% and 41.7%, respectively) were significantly higher than that of the wild type (33.2%). In conclusion, the expression of AmFAD7 was induced by cold and dehydration stresses but inhibited by heat stress. The expression of AmFAD8 was induced by cold stress but inhibited by dehydration and heat stresses. Constitutive expression of AmFAD7 in Arabidopsis increased the membrane stability and freezing tolerance under low temperature stress. However, it also increased the membrane damage and heat sensitivity under high temperature stress.