We calculated the room-temperature phonon thermal conductivity and phonon spectrum of alkyl group-functionalized zigzag graphene nanoribbons (ZGNRs) with molecular dynamics simulations. The increase in both chain length and con-centration of alkyl groups caused remarkable reduction of phonon thermal conductivity in functionalized ZGNRs. Phonon spectra analysis showed that functionalization of ZGNR with alkyl functional groups induced phonon–structural defect scat-tering, thus leading to the reduction of phonon thermal conductivity of ZGNR. Our study showed that surface functional-ization is an effective routine to tune the phonon thermal conductivity of GNRs, which is useful in graphene thermal-related applications.