A new isochrone method of electron spin resonance (ESR) dating has been proposed utilizing the different average dose rates of external radiation for different grain sizes due to the range of external α- and β-rays. The equivalent doses (ED s) obtained by the additive dose method were plotted as a function of the external dose rate (D ex) corrected for the grain sizes. The slope gives an ESR isochrone age since the fracture (T fra) or the surface formation; the coordinate abscissa gives the internal dose (D inT for) where D in is the internal dose rate and T for is the age of the mineral formation. The method has been applied to quartz ( SiO2) grains in geological fault gouge at outcrops of the Rokko Fault giving T fra=102,000 years and T for of nearly zero for grains less than 100 µ m.