We report measurements on the length-dependent thermal conductivity of individual single-walled carbon nanotubes (SWNTs) on a substrate using a four-pad 3ω method with the consideration of heat loss between the sample and substrate. An increase in thermal conductivity with length (0.5—7μm) was observed at room temperature. The length-dependence of thermal conductivity was theoretically predicted by the modified WV model with consideration of second order-3-phonon process. The predicted phonon mean free path is about 175nm. Both the measurement and the prediction show the scale effect of the length on the thermal conductivity of SWNTs.