Abstract: In order to study the electric potential ( EP) distribution characteristics and generation mechanism during rock fracture,the change law and distribution characteristics of electric potential of the crack propagation path were tested with the pre-cracked rock samples. The results show that the tip of the crack and the newly formed crack surface are the dominant positions that the free charges are produced. The strength of electric potential is higher near the crack tip in the propagation path of the crack and lower far from the crack tip;the maximum electric field strength resulted from crack propagation is 0. 42 V / m. In the process of crack propagation,the EP signal polarity was positive and nega- tive alternation,which showed that the property of free charges generated by cracks is opposite and change alternative- ly. The dominant mechanism of free charges and electric potential is the charge separation at two sides of the crack surface of the crack growth. The alternation of charges between positive and negative in the crack surface and the open and close of two crack surfaces during crack propagation resulting in the change of electric field are served as the rea- sons for electromagnetic radiation.