The radiation efficiency, ηR, is an important parameter showing the source property. It is strongly affected by the variation in shear stress with slip. Thermal pressurization is considered to be a significant mechanism in controlling such a variation, thus influencing ηR. In this study, the formula of ηR as a function of slip, δ, on the basis of two end-member models of thermal pressurization, that is, the adiabatic-undrained-deformation (AUD) model and slip-on-a-plane (SOP) model proposed by Rice (2006), is derived. The controlling parameters of the AUD and SOP models are, respectively, δc and L*, which are dependent on thermal, mechanical, and hydraulic parameters of fault rocks. Modeled results suggest that thermal pressurization controls the variation in shear stress with slip and thus influences the radiation efficiency. Results show that ηR increases with δ. The increasing rate of ηR with δ is high at small δ and low at large δ. This indicates that ηR varies very much with δ for small earthquakes and only slightly depends on δ for large events. For the two end-member models, ηR increases with decreasing δc (or L*). When δc=L*, ηR is higher for the AUD model than for the SOP model. The thermal pressurization model is also applied to investigate the shear stress–slip function in a 5 ×5 km square covering a drilled site on the fault plane of the 1999 Chi-Chi, Taiwan, earthquake inferred from seismograms. Results show that the AOD model is more appropriate to describe the inferred shear stress–slip function than the SOP model, and the proposed model is a modified one from the AUD model by including a small amount of loss of frictional heat from the slip zone during faulting.