Four kinds of human IL-12 expression plasmids pcDNA3/p35a, pcDNA3/p35b, pcDNA3/p40a, pcDNA3/p40b were constructed. These recombinant plasmids were co-transfected into Chinese hamster ovary cells and acquired CHO cell lines that could stably secrete IL-12. The highest expressed amount of IL-12 in CHO cell is 105 U/ml. The results showed that the expression of human IL-12 in CHO cell was affected at several levels such as recombinant plasmid structure, DNA intergration, mRNA transcription and protein transpation etc.