Thermal transport issues are one of the major concerns of scientists as the size decreases. To this end, the thermal conductivity of perfect and defective single-walled carbon nanotubes (SWCNTs) functionalized with carbene, which is an important functional group in nanodevices, is investigated. A series of molecular dynamics (MD) simulations are performed and the thermal conductivity is determined by the approach introduced by Muller-Plathe. The results demonstrate that functionalization decreases the thermal conductivity. Also, the thermal conductivity of functionalized SWCNT declines as the weight percentage of functional group increases. Additionally, it is shown that increasing the weight percentage decreases the sensitivity of thermal conductivity. According to the obtained results, simulation temperature does not affect the thermal conductivity of functionalized SWCNTs considerably compared to pure ones. Finally, it is revealed that the presence of vacancy defect reduces the thermal conductivity of functionalized SWCNTs.