The effect of sintering atmosphere on the microstrcture of $SrTiO_3$ has been investigated. A core-shell structure was observed in 1.2 mol% $Nb_2O_5$-doped $SrTiO_3$ after sintering in a reducing atmosphere($5H_2-95N_2$) and then an oxidizing one(air), or vice versa. In undoped, lightly $Nb_2O_5$-doped and $Al_2O_3$-doped $SrTiO_3$ specimens, no core-shell structure was observed. The chemical composition of core and shell region of the 1.2 mol% $Nb_2O_5$-doped $SrTiO_3$ was determined by an electron probe microanalysis(EPMA). The Sr/(Ti+Nb) ratio of shell region sintered in air was about 1% less than that of core region grown in $5H_2-95N_2$. It appears, therefore, that the observed core-shell structure results from the formation of Sr vacancies in an oxidizing atmosphere.
