During early embryo development, Oct-4 is an important transcription factor for the early differentiation. The present study was first examined methylation status in distal enhancer and promoter region of Oct-4 during mouse pre-implantation embryo development. In oocyte and sperm, high methylation was observed in both distal and proximal of promoter in Oct-4. Following fertilization, relatively high methylation level remained until 8-cell stage embryos, but decreased at the morula and blastocyst stage. Specific gene knock down of Oct-4 by siRNA injection into zygote induced higher methylation rates of both distal and proximal region ofpromoter of Oct-4. These results suggest a functional link between the DNA methylation status of distal and promoter region in the Oct-4 gene and the gene sequence-specific transcriptional silencing by exogenous siRNA injection during mouse pre- implantation embryos.