Abstract: The manganese oxide perovskites with different A-site cations are investigated. Largedisorder effects due to size difference between A-site R³⁺ and M²⁺ cations have been found. It’sinfluences on the metal-semiconductor transition temperature are analysed. It is pointed out that this kind of materials should be divided into two parts according to the electron’s bandwidth eg.Some qualitative explainations to this problem are put forward.