In order to investigate the mechanism of the essential role of leukemia inhibitory factor (LIF) in blastocyst implantation, blastocyst was incubated with LIF and its specific antibody. The effect of LIF on matrix metalloproteinases (MMPs) of pre-implantation blastocyst was analysized by RT-PCR and immuno-bloting techniques. The results showed that gene expression and secretion of MMP9 were significantly induced by LIF treatment, after blocking by LIF antibody, those effects were decreased, and the decreasing tendency was weakened with the increase of the incubation time. However the effect of LIF on tissue inhibitor of metalloproteinase1(TIMP1) was not obviously. The results indicate that LIF may play its role by inducing the gene expression and increasing the secretion of MMP9 to facilitate blastocyst implantation.