To investigate the effect of selenium(Se) status on morbidity and process of interstitial pneumonia, we used the Wistar rats as the animal model infected with Mycoplasma Pneumonia (MP). The rats were maintained on the based diets with different doses of Se, infected with MP and exposed to sodium selenite(1.5-2.0 ml/day, containing Se 2 micrograms/ml) or 0.9% NaCl (1.5-2.0 ml/d) by gavage for different time. The experimental rats were divided into five groups, group A, feeding with normal control diet and without infected MP, group B with adequate Se diet and infected MP, group C with Se deficiency diet infected MP and added Se at the same time, group D with Se deficiency diet and added Se after suffered with MP, and group E with Se deficiency diet and infected MP. The results showed that the MP morbidity of the rats fed with high Se diet was significantly lower than that of the rats fed with low Se diet. The pathological change was significant in the low Se group and relatively mild in the high Se group. After infected with MP, the supplementation of sodium selenite to the low Se rats might decrease the MP morbidity and shorten the disease course compared with the group without supplementing Se. The incidence of myocarditis in the infected group with normal Se level diet or in the infected group with Se supplement was lower than that in the group without Se supplementation. The concentrations of Se in plasma and glutathione peroxidase in plasma and white blood cells decreased to a certain level after infection. The present study indicated a positive influence of Se supplement on the morbidity, disease course, and state of interstitial pneumonia in rats.