SiO2 doping in Y2O3 stabilized tetragonal ZrO2 (TZP) materials introduced significant change in mechanical properties around 0.3 wt.% doping level . In order to understand the influence of grain boundary structure and chemistry by doping, high purity undoped and SiO2-doped 3Y-TZP samples were studied using high-resolution and analytical TEM. Typical grain boundary structures are different for the two types of samples, while amorphous film was not observed at most grain boundaries. A new EDS analysis method was introduced to detect the weak Si and Y signals which overlap with the predominent Zr peaks. It revealed that Si segregation to the grain boundary saturates at 12 at./nm2 (or 1.5 monolayer of SiO2) when the SiO2 doping level reached and surpassed 0.3 wt.%. It is the segregated atoms which enhanced the grain boundary diffusivity and therefore altered the deformation mechanism.