Influences on electrical property and phase relationship of oxyapatite-type solid electrolytes based on lanthanum silicate (LSO) were investigated by addition of a small amount of transition metal elements. It was revealed that a very small amount of 3d-transition metal additives into the La-excess-type, i.e., oxygen-excess-type Al-doped LSO, La-10(Si5.8Al0.2)O-26.9, changed its electrical conductivity. Some metals such as iron and chromium showed a positive effect for an increase in conductivity. The specimen with a nominal composition of 0.995{La-10(Si5.8Al0.2)O-26.9}-0.005(FeO gamma), showed the maximum electrical conductivity among the specimens measured in this study. Formed phases of oxygen-excess-type Al-doped LSO based materials were found to be altered even by addition of a small amount of transition metal oxides. La-rich impurity phases such as La(OH)(3) and La2SiO5 are usually detected in the oxygen-excess-type LSO, whereas no formation of La-rich impurity phases was confirmed in the case of iron addition. These results suggest that the enhancement of electrical conductivity by iron addition is due to a disappearance of the La-rich impurity phases as insulators and/or an increase in the concentration of oxygen interstitial carriers within the LSO phase. (C) 2010 Elsevier B.V. All rights reserved.