1. A method for sensing a motion of an object, said method to be implemented by a motion recognition device that includes an image acquiring unit and a processor, and comprising the following steps of: a) configuring the image acquiring unit to acquire continuously a series of image frames by detecting intensity of light received thereby; b) configuring the processor to receive at least one of the image frames and to determine whether an object is detected in said at least one of the image frames; and c) when the determination made in step b) is affirmative, configuring the processor to receive the image frames from the image acquiring unit, and to determine a motion of the object with respect to a three-dimensional coordinate system according to the image frames thus received.