This paper presents a high-resolution microscale ultrasonic particle image velocimetry technique (termed as Micro-EPIV) for measuring multi-component velocity vectors in microscale opaque flows such as blood and biofluid flow in microvessel. The method was tested by in vitro flow imaging and in vivo small animal blood flow imaging studies. The bioflow and blood flow were seeded with ultrasound contrast microbubbles, and were "illuminated" acoustically by 50 MHz and 30 MHz ultrasound, respectively. B-mode images obtained at imaging frame rate of 10 frames per second (fps) and 110 fps were constructed from back-scattered RF signals from bubbles. Then, consecutive images were processed with optimized PIV algorithm, to acquire multi-component velocity vectors. The results were in good agreement with analytical solutions and the velocities measured by ultrasound Doppler technique.