Manganite ceramics with chemical composition of La_0.6Sr_0.4MnO₃ were sintered with SiO₂, V₂O₅, Bi₂O₃ or CuO as additive. The manganites with additives were decomposed partially and their TC were changed from that of the manganite without additives. Temperature dependence of magnetoresistance was changed remarkably. The changes in magnetoresistance were supposed to be not due to the change in chemical composition of the manganites but also due to change in microstructure of the manganite ceramics.