Abstract: Transgenic plants with high and stable expression are ideal to ge t by plant gene engineering, but gene silencing limits their application. The mechanisms of gene silencing include the ectopic pairing among multi_copy of genes, methylation on the sequences of genes, structure changing of chromosome on insertional locus, and the declining control of post_transcription. Studying the cause for silencing of foreign genes and learning the method to overcome gene silencing are important for the development of plant gene engineering.