The conducting mechanism of antimony doped tin oxide is sumed up. Oxygen vacanices and dopants,Sb5+,supply n-type charge carries to the conductibiliity of SnO2 are two conducting mechanisms. Using the formula of the materials conductibility,this paper also proves that there exists the optimum content of the Sb in ATO. Based on the exisitent mode,this paper also proves there is conductibility upper limit of ATO,the optimum dopant,Sb,in ATO is 1 49%(quality percent),the theory max conductibility of ATO is 0...