Linoleic acid (C18∶2n-6) and α-linolenic acid (C18∶3n-3) are found widely in fungi, plants and some lower animals. However, they can not be synthesized in mammals due to lack of △12 and ω-3 fatty acid desaturases. To enable endogenous production of essential fatty acids in mammalian cells, here the stable expression of a Caenorhabditis elegans gene FAT-2 encoding △12 fatty acid desaturase in CHO cells was reported. First, the FAT-2 coding sequence was cloned by RT-PCR. To facilitate high level synthesis of heterogeneous protein, the codon usage of the fatty acid desaturase genes was optimized according to the codon preference of mouse by site-directed mutagenesis, 2 synonymous mutations were introduced into FAT-2 gene by overlapping PCR. The codon-modified gene was finally fused to pBudCE4.1 vector (Invitrogen) under the control of CMV promoter. The expression vector pBudCE-FAT2 was linearized with NheⅠ, and then transfected CHO cells, the cells were under Zeocin selection for nine days and then propagated, then the transfected cells were harvested. The genome and total RNA were isolated for PCR and Norhern blot ananlysis. The results revealed that FAT-2 gene has been integrated into the genome of CHO cells and expressed properly. Fatty acids of total cellular lipids were analyzed by gas chromatography. The results indicate that the expression and function of △-12 fatty acid desaturase resulted in accumulation of linoleic acid. The levels of linoleic acid in transgenic cells were 2.4-fold higher than those in wild-type cells. The moderate linoleic acid in CHO cells was derived from cell culture media uptaken by cell membrane. The results demonstrate that a heterogenous desaturase gene can function well in mammalian cells and prove that transgenic approach is an efficient strategy for changing fatty acid composition of mammals.