High density (∼96%) garnet-type Al-contained Li6.75La3Zr1.75Ta0.25O12 (LLZTO-Al) solid electrolytes are prepared by conventional solid-state reaction and the following flowing oxygen sintering process. An overall ionic conductivity as high as 7.4 × 10−4 S cm−1 at 25 °C is achievable, remarkably higher than that obtained by sintering in other atmospheres. The dependence of density and conductivity of solid electrolytes on sintering under different oxygen partial pressures is discussed. Atmosphere sintering is proved to be an effective method to improve the relative density of lithium oxide ceramics.