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In this paper, the significant electromechanical properties of the novel high-temperature piezoelectric single crystals YCa4O(BO3)3 (YCOB) were fully characterized over the temperature range from room temperature to 800 °C, including the dielectric, elastic, and piezoelectric properties, and electromechanical coupling coefficients and mechanical quality factor. More importantly, the complex coefficients were completely determined which take into account the mechanical and electrical losses of the material and, therefore, can describe the crystal behavior more accurately. The complete measurement procedures and calculation methods required to obtain all of these parameters were proposed. YCOB resonators with 23 different types of orientations were designed and measured, and all of the independent dielectric, piezoelectric, and elastic coefficients (4, 10, and 13, respectively), 6 important coupling factors, and the mechanical quality factor were determined. It was found that (1) the dielectric permittivity and loss increase dramatically and nonlinearly with the rise of temperature; (2) the elastic compliance constants basically increase linearly as the temperature rises, while the stiffness constants decrease linearly; (3) the piezoelectric charge constants d i j increase slightly with the elevated temperatures; (4) the coupling factors and mechanical quality factor decrease linearly with an increase of the temperature, and the coupling factors are in the range of 0.005–0.052, and the quality factor drops from 10 300 to 1300 with the temperature increasing from room temperature to 800 °C. [ABSTRACT FROM AUTHOR]