Abstract: Objective Constructing the eukaryotic expression vector of mouse LIF and conducing to inprove the directional differentiation research of ES cell about other laboratory animals. Method Mouse secretedLIF with and without signal peptide were cloned from mouse livers by RT-PCR, and then subcloned into pBS-T vector and pMD18-T simple vector. Digesting fragments were recovered and inserted into pSecTag/Hygro with molecular cloning technique, and digested analysis by restrictive enzymes. Results By digestion identification with restrictive enzymes the LIF gene was cloned into eukaryotic expression vector pSecTag/Hygro successfully and the exp. ession vectors we constructed were called pSecTag-lif（sp⁺） and pSecTag-lif（sp ）. Conclusion It is valid to construct the eukaryotic expression vector of mouse LIF, and this not only establishs the base of molecule principle that maintains the undifferentiated ES cell, but also can provide a new method for the research of transgenic laboratory animals.