Abstract: Selenium ( Se) is one of the important and essential trace elements in human body, but the range between biological activity and toxicity of Se is extremely narrow, so it's easy to show its toxicity.However, nano- Se has an advantage of good security and efficient.In recent study, sodium selenite was reduced to nano- Se by LA4 strain which had high tolerance with sodium selenite. The nano- Se were given to Kunming mice by gavage 0, 0.2 ( low dose group) , 0.5 ( middle dose group) and 1.0mg / kg·bw ( high dose group) nano- Se respectively. The effect of LA4 lactic acid bacterially synthesized nano- Se on oxidation resistance of mice was researched. The result of Scanning electron microscopy- energy dispersive X- ray spectroscopy ( SEM- EDX) showed that sodium selenite was reduced into spherical particles in nanoscale ( about less than200nm) by LA4 lactic acid bacteria. LA4 lactic acid bacterially synthesized nano- Se could improve the activity of glutathione peroxidase ( GSH- PX) of whole blood and testis in mice ( p < 0.01) obviously, and also improve total antioxidative capacity ( T- AOC) and peroxidase ( POD) of important organs, such as brain, liver, spleen and so on, in mice ( p < 0.01) obviously.Moreover, LA4 lactic acid bacterially synthesized nano- Se could enhance the activity of total superoxide dismutase ( T- SOD) of serum in mice ( p < 0.01) , it also could reduce the content of malondialdehyde ( MDA) of serum and important organs in mice.In a conclusion, LA4 lactic acid bacterially synthesized nano- Se couldimprove the oxidation resistance of mice, and the oxidation resistance of mice enhanced with the content of LA4 lactic acid bacterially synthesized nano- Se increasing. But when the content of LA4 lactic acid bacterially synthesized nano- Se increased to1.0mg / kg·bw, also have oxidation resistance