The irradiation of ultrasonic waves to liquid causes liquid flow toward the propagation direction of the ultrasonic waves. This paper proposes the application of this phenomenon, called acoustic streaming, to the development of micro-actuators such as micro-manipulators for small particles and micro-pumps for liquid. Since high-frequency ultrasonic waves induce acoustic streaming very efficiently only in the vicinity of the transducer, micro-manipulators are shown to be realised by using a two-dimensional ultrasonic transducer array and controlling electrical power supplied to each transducer. It is also concluded that micro-pumps could be developed by combining a transducer array with a topographic channel structure for liquid flow.