In this study, the influence of alkyl functional groups on thermal conductivity of graphene oxide (GO) nanosheets is studied using a reverse non-equilibrium molecular dynamics simulation. The different types of alkyl-functionalized GO with functional groups containing six, twelve, sixteen and eighteen carbon atoms are investigated while the functional groups were bonded to the GO nanosheets both covalently and non-covalently. The objective is to investigate the effect of functional groups length as well as functionalization mechanisms on thermal conductivity of nanosheets. The results demonstrate that the thermal conductivity of GO reduces after alkylation and decreases with an increase in the chain length of functional groups. The analysis of the phonon spectra demonstrates that the functionalization of GO disrupts its phonon vibrations which results in a reduction of the thermal conductivity. Furthermore, the investigation of the alkylation mechanisms shows that the major portion of the reduction in the thermal conductivity is due to the non-covalent functionalization. This study can provide helpful information for controlling the thermal properties of alkylated GO for diverse applications such as superhydrophobic coatings, polymer composites, and nanocolloids. It also enhances the understanding of the mechanisms which affects the thermal properties after functionalization process.