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The robotic fish that utilize the body/caudal fin undulatory locomotion has long interested both biologists and engineers. Although a variety of free swimming robotic fish prototypes have already been developed, very few studies addressed the methods for determining quantitative thrust efficiency. In this paper, we propose a novel experimental method that enables the simultaneous measurement of the power, wake flow field, and self-propulsive speed of a robotic fish, which together facilitate a quantitative measurement of its efficiency. Our results show that the optimal thrust efficiency of the robotic swimmer is within the Strouhal number (St) range of 0.3 ≤ St ≤ 0.325 when single-row reverse Karman vortices are produced. Nevertheless, present robotic fish swam at Strouhal numbers outside the optimal region under self-propulsive condition, and produced another type of wake structure: “double-row vortices.” We also show that robotic fish that utilize a low amplitude with a large flapping frequency produce higher self-propulsive speeds, whereas a larger amplitude paired with lower frequency results in higher efficiency. Additionally, a peak efficiency value of 31.6% is recored for the self-propulsive robotic swimmer. The general applicability of this experimental method indicates that broader issues regarding thrust efficiency for biomimetic underwater propulsive robots can be quanlitantively measured.