The effects of WO 3 addition and sintering condition on the microstructure and electrical conductivity of SrTiO 3 ceramics have been investigated. When P O 2 ≤ 10 - 1 0 atm, the room temperature electrical conductivity was proportional logarithmically to WO 3 content. Whereas when P O 2 > 10 - 1 0 atm, the electrical conductivity increased with WO 3 content up to 1.2 mol%. When the WO 3 content is higher than 1.2 mol%, the electrical conductivity decreased. The decrease of conductivity seemed to be due to the formation of Sr vacancies. Sr vacancies are considered to be formed by the movement of Sr ions into the grain boundaries. In 3.0 mol% WO 3 -doped specimens, Sr-W-rich phases have been observed by SEM and EDAX analysis.