Semiclassical Marcus theory has been widely used in evaluating molecular parameters and predicting the charge mobility for organic semiconductors.In such approach,there should be no isotope effect for mobility since both electronic coupling and charge reorganization are independent on isotope.Here,we point out that since the vibrational modes responsible for charge transfer are of high frequency,the nuclear tunneling effect is important and by using a quantum charge transfer formula,we predict that isotope effect tends to reduce the charge mobility in organic materials.This is because that nuclear tunneling tends to favor charge transfer rate while heavier nucleus decrease quantum effect.Our calculations focus on the n-channel materials,alkyl substituted naphthalene diimide(NDI)and perylene diimide(PDI),by deuterium and 13C isotopic substitution.For N,N-bis(n-hexyl)-NDI,we show that by all-deuteration on alkyl side chains and all 13C-substitution on backbone,the room temperature electron mobility is decreased by ca.18%and 7%respectively.While by all-deuteration on NDI backbone,mobility is decreased less than 1%.Similar isotope effects are found in N,N-bis(n-octyl)-PDI.However,there is nearly no isotope effect for the all-deuterated rubrene,in agreement with experiment.It is found that isotopic effect on charge transport only occurs when the substituted nuclei contribute actively to vibrations with appreciable charge reorganization energy and coupling with carrier.