A method of specifying particle size distribution in fluorocarbon emulsion by means of stepwise centrifugation is proposed. On account of very large specific density of fluorocarbon, a physical calculation was made to classify particle size (the maximum and average) of fluorocarbon emulsion in correlation with centrifugal rotation applying Stokes’ law of sedimentation. Samples of FC emulsion were made with different grade of emulsification, and distribution of particle sizes were examined by the proposed method using centrifugal differentiation and electron microscopic photography. Close coincidence between the findings by both methods was found, justifying the centrifugal method. Simple expression to specify status of ‘particle-size pattern’ (PSP) in FC emulsion is also proposed. When each of 5 different samples having different PSP was injected into animals, PSP of the emulsion was found to be closely related to survival of rats or mice, i.e., the higher average particle size and the broader PSP, the higher toxicity. It is concluded, therefore, that fluorocarbon emulsion should consist mainly of particles smaller than 0.1 µm as a candidate for red cells substitute.