A designer of marine propellers estimate the maximum revolutions free from cavitation in accordance with the criterion of cavitation, and when that allowable revolutions are lower than the required revolutions they give the propeller wider blades in order to raise its cavitation limit to a higher value of revolutions. Eggert's formula is used generally for this purpose. But in spite of these efforts the phenomena which look like the proof of formation of cavities, i. e. the erosion and blade pitting, occur oftentimes in the most of recent propellers working at high load and high revolutions. This cause is the fact that Eggert's formula gives too lenient revolutions at which cavitation begins. The author has discussed the characteristics of cavitation which appears on propeller blades by the approximate formulae deduced from the newest vortex theory and the results of Gutsche's experiments. Then he has devised a new criterion of cavitation; which can be used easily by propeller designers and may be more accurate than the older ones. The author concludes as follows in consequence of these studies: 1) It is effective for the prevention of cavitation to design a propeller blade with aerofoil sections near the root and with ogival sections near the tip. 2) The section at r/R =0.7 must have the form of aerofoil, because the ogival section at this radius forms the face cavitation at comparatively low revolutions. 3) It is necessary to choose not only the proper blade thickness ratio but the proper pitch and diameter in order to design a marine propeller free from cavitation at high revolutions.