In this paper, we tried to evaluate several small ZnO single crystals grown by the hydrothermal method using a micro-line-focus-beam ultrasonic material characterization (LFB-UMC) system. We measured leaky surface-acoustic-wave (LSAW) velocity distributions at 225 MHz for the top and bottom surfaces of three (100) (Y-cut) ZnO specimens with different colors, i.e. different electrical properties. We prepared a standard specimen of YX-ZnO, proper for calibrating the LFB system, which is independent of the piezoelectricity of ZnO. We evaluated the Y-cut specimen surfaces using the piezo-active Z-axis LSAW propagation, resulting in a high velocity of 2649 m/s for a very resistive (> 100 Ωm) surface and a very low velocity of 2618 m/s for a very conductive (< 1 Ωm) surface. Additional experiments for cross-sectional ±c-plane surfaces of one Y-cut ZnO specimen plate revealed significant LSAW velocity distributions associated with the crystal growth condition changes. This ultrasonic method will be useful for further understanding the crystal growth processes.