To measure intracardiac and intravascular pressure, we have studied a new technique based on the principle that an elastic microcapsule in liquid changes the resonant frequency depending on the surrounding pressure. In this paper, we investigated the pressure dependent changes in frequency spectrum of ultrasound passing through elastic microcapsules which were made of vinylidenedichloride-acrylonitrile copolymer. To avoid measurement error due to floating of microcapsules, we set a microfilm (0.15 micron in thickness) in an airtight cistern filled with degassed water, and attached microcapsules on it. Transmission wave, passing through the elastic microcapsules, was received and digitized with 10 bit A/D convertor at radio-frequency sampling rate. The cistern was pressurerized from 0 to 300 mmHg using sphygmomanometer. Following results were obtained: 1) The frequency spectra of transmitted ultrasound showed marked attenuation, resulting in dips. They demonstrated resonant frequency of 100 kHz, 200 kHz, 300-400 kHz at diameter 20-32 μm 45-63 μm, 74-90 μm, respectively. The characteristic dip of the frequency spectrum, indicating resonant frequency, was shifted to the higher frequency in the order of the diameter of the microcapsules. 2) The amplitude change in the frequency spectra linearly increased with the pressure. 3) Pressure dependent resonant shift of microcapsules were clearly observed. The present ultrasound technique using newly developed microcapsules may have a potential to measure the pressure in the heart and vesseles.