Novel diagnostic method named 'sonocytometry', in which streaming blood cell is diagnosed by the reflection of high frequency ultrasound from the cell, is proposed. In the present study, the differentiation of the particle size is performed as a basic study on sonocytometry. Ultrasonic backscatter signal from either 80 or 100 μm diameter polystyrene particles was measured by an ultrasonic transducer with the central frequency of 30 MHz. The spectrum of the reflected signal showed different characteristics according to the particle diameter. Theoretical value of backscatter was calculated by Faran-Hickling model and the correlation coefficient of measured and theoretical value by varying the spherical diameter showed the local maximum value at either 80 or 100 μm diameter. The principle was also validated on the streaming particles in a flow channel. The method successfully classified the particle size. Sonocytometry would be clinically applied for diagnosis of malaria or leukemia.