A micro bar code and a recognition system and method thereof are provided. The micro bar code is provided with a plurality of nanounits disposed in a predetermined arrangement. The nanounits are provided with nanomolecules having different sizes respectively. In the recognition system and method, a sensing module senses the arrangement of the nanounits and the sizes of the nanomolecules on the micro bar code. The sensing result from the sensing module is transmitted to an operating module and then inputted to a bar code database that stores information corresponding to arrangement of each set of nanounits and sizes of each set of nanomolecules, so as to recognize the information on the micro bar code to be recognized. Finally, the recognition result is transmitted back to the operating module for recording. The micro bar code has advantages of small size and large data storage capacity.