Transgenes are inactive and the trans-gene expression couldn't be detected thoroughly ,which are called as gene silencing in transgenet-ic plants. The main mechanisms, including position effects, transcriptional silencing and post-transcriptional silencing was presented, the means of stabilizing gene expression in transge-netic plants was discussed. 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.