We use multiscale modelings to investigate the heat dissipation in 2D transistors based on phosphorene and silicene. First, molecular dynamics (MD) simulations were used to calculate the thermal interface resistance R int between the 2D materials (phosphorene and silicene) and dielectrics substrates (SiO 2 and TiO 2). The calculated R int of these systems are close to that between graphene and SiO 2 and are insensitive to the temperature. The MD values then served as inputs for finite-element simulations at the device scale. It is found that the heat-dissipation ability of the 2D transistors can be improved by increasing the thermal conductivities of the 2D materials as well as of the substrate. However, in contrast to the common belief, it is difficult to largely reduce the hot-spot temperature by tuning the interface thermal resistance. Finally, we show that the cooling performance of silicene/SiO 2 system can be significantly improved with a few-layer graphene heat spreader. These results provide important information for the novel design of 2D transistors in terms of thermal management.