Abstract: The shape of load-loading displacement curve of straight notched 3pb test is analyzed. According to the theory of stiffness, the loading-unloading cycle after failure load can be obtained from ordinary testing machine, and which is proved in the test. Using the standard short rod specimen 50mm in diameter of marble and granite, the influence of consecutive loading-unloading cycles to rock Kic was studied. It was found that the consecutive loading-unloading cycles reduces the value of rock Kic Testing specimen with four kinds of rock were studied on the "plasticity" influence of straight notched 3pb specimen. It was concluded that "plasticity" influence exists in straight notched specimen and the method for "plasticity" correction was given.