P>Objective To study the temporal and spatial expression of Nanog in developing primordial germ cells (PGCs) of mouse. Methods Immunofluorescence technique was taken to qualitatively analyze the expression of Nanog in PGCs of 7.5days(post-coitus days)-15.5days mouse embryo. A further quantitative analysis of Nanog expression change in PGCs of 11.5days-15.5days mouse embryo was done by using SSEA-1 and Nanog double labelling immunofluorescence staining with confocal microscopy. Results The PGCs in mouse allantois,hindgut,dorsal mesentery and genital ridge were Nanog positive. Both of the highest positive ratio and the highest fluorescence intensity appeared in PGCs of 11.5days mouse embryo. For the female mice, Nanog expression drops dramatically after 12.5days, and for the male mice, a noticeable decline of Nanog expression was occurred after 13.5 post-coitus days. Conclusion Nanog expresses stably in the proliferating PGCs. The downregulation of its expression may be related with the differentiation of PGCs./P>