Abstract: A three-dimensional (3D) motion identifying method and system are used for identifying a motion of an object in a 3D space. First, the method provides a database recording sets of predetermined inertial information, and each of the sets of predetermined inertial information is an inertial movement of a specific motion in the 3D space. Then, inertial information of the object in moving is retrieved via a motion sensor in the object, and the inertial information is compared with all predetermined inertial information in the database to determine similarities therebetween. Finally, whether the motion of the object is the same as any predetermined inertial information or not is determined according to a degree of the similarity. As a result, more complicated motions of the object can be directly identified via the comparison with the database.