Fixed superiorities of dental porcelain have been evaluated in many respects since it was introduced in dentistry almost a century ago. It seems, however, that porcelain has been used less frequently for crown and bridge work than dental resins. One of the causes for it may be the difficulty of porcelain to successfully compensate for its inherent large shrinkage during firing. For the best clinical performance with porcelain, it would be essential to complete the whole process in a way to minimize the shrinkage and porosity. In the present paper, occurrences of shrinkage and porosity of dental porcelain were investigated in relation to thickness of porcelain, kind of mixing medium, condition of condensation, firing atmosphere, firing temperature and holding period at a particular temperature. The material used for all the experiments was one and the same kind of air-firing porcelain. The results were as follows : 1. A mechanical condenser was experimentally made, in which the vibration is generated by the deviation from the uniform revolution of a round plate connected to the axis of the motor. In any thickness of the specimen, the void space was reduced, or the degree of condensation became higher, as the number of the motor revolution increased within a certain limit. 2. When the specimen was fired at the temperature recommended by the manufacturer (900℃) in air or in vacuum, firing shrinkage decreased significantly with an increase in the degree of condensation, while no changes were found in porosity. 3. Under the same degree of condensation, both shrinkage and porosity generally showed a tendency to increase with an increase in thickness of specimen. 4. There were no statistical differences in shrinkage and porosity between the specimens fired in air and in vacuum at the temperatures not higher than 900℃. 5. When the firing process was carried out at the temperatures higher than 900℃, vacuum-fired specimen showed higher shrinkage and lower porosity than air-fired one. 6. In comparison of the effect of firing temperature on the shrinkage and porosity, shrinkage increased and porosity decreased significantly in vacuum-firing, as the firing temperature increased above 900℃. In air-firing, on the other hand, both shrinkage and porosity appeared constant above 900℃, probably because the residual voids inside the specimen were sealed by the glazed surface. 7. As the holding period was prolonged at 900℃, the firing shrinkage slightly increased and porosity slightly decreased both in air and in vacuum.