The shock model of gamma-ray bursts (GRBs) has two free parameters: ϵ B and ϵ e . It has been shown that ϵ B should range between few×10 −3 and few×10 −4 . However, how to calculate the value of ϵ e has remained an outstanding theoretical problem for over a decade. Here, we demonstrate that the Weibel theory inevitably predicts that . The GRB afterglow data fully agree with this theoretical prediction. Our result explains why the electrons are close to equipartition in GRBs. This ϵ e – ϵ B relation can potentially be used to reduce the number of free parameters in afterglow models.