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The microbubble ultrasound contrast agent (UCA) has been widely recognized as a potential noninvasive tool for blood pressure measurement. However, UCA indices such as the shift in the resonance frequency and echo amplitude have problems of low resolution, nonlinear relationship with blood pressure, etc. In this paper, a novel UCA index, the shift in the subharmonic optimal driving frequency (SSODF) of microbubbles, is proposed. The effectiveness of the index for estimating blood pressure was evaluated by performing a microbubble acoustic response simulation. The behavior of commercial UCA microbubbles was investigated as a function of the driving acoustic pressure (in kilopascals) and ambient overpressure (in millimeters of mercury). Simulation results showed that for a 1.6-μm-diameter microbubble, SSODF increased linearly with the overpressure in a range of 0 to 200 mmHg and was maximum (2.07 MHz) at 380 kPa. Changes of the overpressure as small as 5 mmHg can be detected using SSODF. For a population of microbubbles with a Gaussian size distribution (mean diameter: 1.6 μm, standard deviation: 0.2 μm), SSODF was 1.7 MHz at 280 kPa. With further experimental validation, the proposed method may be developed as a novel noninvasive technique for accurate blood pressure measurement.