Human G-CSF genomic DNA was obtained by PCR and mammary expression vector was constructed using WAP gene 2.6 kb promoter as control element. Two transgenic mice were produced by microinjection method and identified by PCR and Southern blot. G-CSF expression level was as low as 120～250 μg/L in transgenic mice milk. In order to study the cause of its low expression, human G-CSF gene was amplificated by RT-PCR in mammary gland of mice. Sequence analysis showed that this gene missed the fourth exon that was recoginsed as the intron due to RNA abnormal splice. It is possible that RNA abnormal splice lead to low expression in transgenic mice.