A circuit substrate includes a core substrate having cavity, metal blocks in the cavity, first and second build-up layers including insulating layers and laminated on first and second sides of the core substrate such that the insulating layers are covering the cavity, and a filling resin filling gap formed between the cavity and metal blocks in the cavity. The cavity is penetrating through the core substrate, the core substrate has an intracavity projection structure projecting from one or more side surfaces of the cavity such that the projection structure is positioned between the metal blocks and separating the metal blocks from contacting each other in the cavity, the first build-up layer has first conductors connected to the metal blocks such that each first conductor conducts electricity or heat, and the second build-up layer has second conductors connected to the metal blocks such that each second conductor conducts electricity or heat.