In order to clarify, factors affecting on mechanical strengthof porcelain bodies, some experiments were carried out for bobies consisting of Hadong kaolin, quartz of optical glass grade and potash-feldspar. At first, degree of vitrification of the fired bodies were examined from the results of bulk density measurement. Constituent minerals of these fired bodies were mainly examined by means of X-ray diffraction, and their microstructure were observed by means of the polarization microscope and scanning electron microscope. Transverse and impact strength of the bodies were also determined. Finally, relations between their mechanical strength and the other properties obtained were discussed. The mechanical strength, at least the transverse strength increased with increasing densification of the bodies and accompanied with an increase of stress produced in the quartz grains and, perhaps other phases composing the bodies.