In order to understand the mechanism of melt generation and the origin of high heat flow in subduction zones, a series of numerical models of the thermal and flow structures in the mantle have been tested in two‐dimensional boxes with an inclined subducting slab of constant velocity. In contrast to previous models, (1) large convecting cells are used without imposing a high‐temperature profile on the backarc boundary, which enables us to discuss the global heat balance and to seek for the heat sources responsible for the melting and high heat flow observed in subduction zones, and (2) various mechanical conditions (e.g., coupling between the slab and overlying mantle wedge, buoyancy associated with melting) and heat sources (e.g., heat flux from below, internal heat generation, viscous heating) are tested in varying proportions. In all the calculations, steady state or near‐steady state with a small instability periodically occurring at the upper thermal boundary layer is achieved. Under these conditions, the global heat balance can be described by a simple boundary layer argument. The results show that, in order to attain a high enough temperature for melting and a high heat flux, a large amount of internal heating (i.e., more than 2.5×10−7 W/m3) is required if the convection is limited within the upper mantle. The high internal heating required may be explained if the radioactive nuclides in fluids expelled from the subducting slab are added to the wedge and circulate in the convection cell for a sufficient time. Another possible explanation for melting and the high heat flux is that the hot material is supplied from the lower mantle. Based on the thermal and flow structures obtained, melting regimes in subduction zones are discussed, in which the following key processes take place: (1) melting associated with pressure change of a rock packet with its movement, especially compression melting in the downward flow of the mantle wedge along the slab, and (2) melting due to compositional change of the rock packet associated with migration of H2O and melt. To solve these problems in detail, further studies on distribution and migration of the fluids will be required.