Abstract: Different levels of sodium selenite was added in the diet of cocks to study the sediment amount of selenium in the myocardium tissues of cocks and its effects on the structure of myocardium tissues of cocks, so as to provide reference basis for the application of sodium selenite in the poultry production. 80 health Hyline White adult cocks with similar body weight were randomly divided into 4 groups to add 0, 0.3, 0.6 and 1.0 mg/kg sodium selenite in the diet. The myocardium samples were collected to determine Se content at the end of the experiment. And structure of tissue cells was observed by HE staining. The results showed that Se content in myocardium tissues increased with the increase of Se content in the diet. This study indicated that Se content in myocardium tissues gradually increased with the increase of Se level in the diet. High-selenium diet had some effects on the structure of myocardium tissues. However, the effect was unconspicuous because of lower sediment capability in myocardium tissues than other tissues.