In order to study grain boundary's (GB's) blocking effect in lanthanum silicate electrolyte, high density Al-doped apatite-type lanthanum silicate was synthesized and characterized by Impedance spectroscopy, scanning electron microscopy, X-ray diffraction and transmission electron microscopy. Microstructural characterization indicated GB's blocking effect was an intrinsic effect. Further microanalysis shows that GB region is rich in La and poor in Si in comparing with grain interior (GI). Our discussion suggested that the chemical variation from GI to GB, on the one hand, could degrade GB region's conductivity; on the other hand, it introduced a strong space charge effect at GBs. The latter was believed to play a dominant role in GB's blocking effect.