The Hokyo originally consisted of sixteen iron plates hung in a frame in two rows which formed a certain series of notes and it was beaten with two small sticks. The Imperial Treasury of Skiosoin has preserved nine such plates (length of 10.5-15.1cm.) each in the shape of a rectangular plate with an arched top. Recently the Hokyo was reconstructed in the original form. By means of the sound spectrograph (Sonagraph), the sound of the nine iron plates of the Shosoin Hokyo and all the plates of the reconstructed one was analized. The special characters of the tone colour of each plate were cleanfled by examining the distribution of its partial tones and noises recorded in the spectrogram patterns. Also the time variation of sound intensity of the plates was inquired by surveying the amplitude of the sound waves recorded by electromagnetic oscillograph. The distributions of the nodal lines in the vibration of a plate was obtained by means of the resonance method. Also theempirical formula representing the relationship between the frequency of the fundamental tone of the plate and its length and thickness were derived. Finally, the method of tuning of the kyo plate was discussed, and the rate of change in pitch corresponding Htoo the decrease in the length and thickness of the plate was acquired.