The friction experiments on macroscopically homogeneous and non-homogeneous faults were performed by using a double-shear friction rig, and the waveforms of acoustic emission and the fault slip corresponding to the stick-slip instability events were analyzed. The results indicate that there exist two types of nucleation phase for stick-slip instability of non-homogeneous faults, and one is coincident with the preslip model, the other with the cascade model. It is concluded that the seismic nucleation phase exists objectively, and its generation is related to heterogeneity of fault. The friction experiments on macroscopically homogeneous and non-homogeneous faults were performed by using a double-shear friction rig, and the waveforms of acoustic emission and the fault slip corresponding to the stick-slip instability events were analyzed. The results indicate that there exist two types of nucleation phase for stick-slip instability of non-homogeneous faults, and one is coincident with the preslip model, the other with the cascade model. It is concluded that the seismic nucleation phase exists objectively, and its generation is related to heterogeneity of fault.