Effects of grain size of quartz mixed to porcelain bodies on the mechanical strength of the fired bodies were already reported. In the present paper, effect of amount of the quartz on the strength was examined. There are no relation between content of mullite of the fired bodies and their bending strength, and amount of addition of quartz into the bodies and the strength. But for amount of quartz remained in the fired bodies, the bodies containing a few to 35% of such a quartz have as high bending strength as 170-180MPa. If the content increased beyond this range, their strength decreased. The bending strength of the fired bodies increased with increase of unit cell volume of quartz in the bodies. This result confirms that the prestress effect generated in the glassy phase surrounding the quartz grains is one of the most important factors raising their mechanical strength of the porcelain bodies. Increase in quartz content in porcelain bodies raises their maturing temperature comparatively, but thermal expansion coefficient of these fired bodies which showed highest strength in each body series, did not differ so much.