Abstract: Based on the Dieterich seismicity theory, this study derived a general formula used to calculate decay rate and cumulative frequency of aftershocks following a mainshock, of which the shear stress variation in the fault plane follows the Jeffreys-Lomnitz creep law and is directly related to the modified Omori law. The p value in the modified Omori law is positively correlated with post earthquake loading and unloading process of the transient stress. Combined with the postseismic slip model proposed by Rubin and Ampuero, this study obtained approximate formulas to describe the variation of seismicity and to fit aftershock sequence following the 2008 M_W7.9 Wenchuan earthquake. The results show that the p value is directly associated with the ratio of b/a, where a and b are the frictional parameters used in rate- and state-dependent friction, and the c value used in the modified Omori law is associated with D_c, a critical creep distance described by rate- and state-dependent friction. For the M_W7.9 Wenchuan earthquake sequence, the fitting results indicate the ratio of b/a and D_c are about 1.13 and 2 to 3 cm, respectively. It needs to be emphasized that postseismic creep proposed by Rubin and Ampuero is an inherent stage during earthquake cycle, which describes characteristics of the transient velocity change in seismogenic zone after the mainshock.