Abstract: This paper proposes a conceptual model of earthquake source body with Kelvin viscoelastic property to investigate the physical mechanism of Omori-Utsu’s law, supposing that tectonic stress field after main shock does not change with time and equivalent viscosity in the aftershock region is much lower than that of its outside in the period of total aftershock activity. This model can simulate aftershock sequence induced by post-seismic creep and stress readjustment, and the whole process including creep stopping, materials recovering to its elastic state, and faulting turning to stick state for next earthquake. Finite element method is used to calculate stress field evolution caused by a main shock and its aftershocks in the model with heterogeneous material properties. Further- more, the model and the method are used to simulate decay of the aftershock frequency of the 1976 M_S7.8 Tangshan earthquake. The results show that the mechanism of Omori-Utsu’s law may be attributed to the stress changes caused by the creep of the fault and earthquake source body, which implies that aftershock frequency depends on the creep rate and decay time of the aftershocks is controlled by the equivalent viscosity. The lager the viscosity is, the longer the creep time or the aftershocks last.