Heat conduction is of great importance in thermal design for high power diode lasers. In this paper, an analytical, three-dimensional, steady-state, multilayered, thermal model for a high power diode laser is derived. The temperature and heat flux distribution are discussed for an epi-down bonded broad-area diode laser, and it is found that heat spreading within laser chip contributes 6.8% to total heat dissipation. Further discussion is carried out on heat flow in the submount to show the submount size requirement for which this model can be used. Simulation result based on finite element method (FEM) is employed to confirm the calculation accuracy from this analytical model.