Yak is a very useful animal. The cDNA library of yak liver was constructed, which made a preparation for further cloning and expressing of wanted genes.mRNA from yak liver was extracted and purified, and cDNA was obtained by reverse transcription,and then cDNA library was constructed using plasmid pSPORT1 as the vector. The volume of the library was 1 x 10(6) transformants/microg cDNA and the rate of recombination was 93%. The results indicated that the cDNA library had enough volume for selecting wanted genes and made the basic of studying yak's genetic background.