Selenium is one of essential trace elements in organism． In the process of livestock and poultry breeding，adding selenium to the feed can obviously improve the production performance，antioxidantfunction and immune function of livestock and poultry． The effects of selenium on development of immuneorgans，immune response and mechanism of selenium promoting immune function were summarized inthis article． The way of action of selenium in livestock immune system and the selection of selenium fromdifferent sources were also prospected．