Abstract: The whole fracture process was analyzed by three-point-bending tests on lightly-reinforced concrete beams with different initial notch lengths. The state of cracking initiation was determined experimentally. The mechanism of the first critical state in the load-displacement curve was discussed. Furthermore, a relative ratio of reinforcement is proposed based on comparison between the critical load and the load-carrying capacity of beams. Thus, the ultimate failure mechanism of lightly-reinforced concrete beams was analyzed and the Model I crack propagation of concrete under the bridging action of steel bar was determined.