This paper aims to computationally investigate the thermal effect, combining the internal heat and the ambient temperature, on the lithium (Li) dendrite growth process. To achieve this, the recently developed phase-field Li-dendrite model is further extended by coupling with a heat transfer model. The two models are linked via a temperature-dependent ion diffusion coefficient to investigate the evolution of the morphology and size of dendrites. Three levels of cases are used to progressively investigate the thermal effect on Li dendrites: (1) uniform ambient temperature, (2) temperature gradient along the charging direction, and (3) internal heat induced spatially distributed temperature. The results show that the normalized dendrite length decreases as the ambient temperature increases, which agrees well with the published experimental measurements. The temperature gradient is applied in the 2-D system showing that the formation of lateral branches can be prevented with the presence of the temperature gradient. The third case shows that the temperature significantly increases at the dendrite-electrolyte interface, and the dendrite deviates from the tree-type shape to a nearly rhombic shape. The simulation results provide valuable bases for the future comprehensive studies of the temperature dependent Li dendrite growth process.