A circuit board includes a composite structure layer, at least one conductive structure, a thermally conductive substrate, and a thermal interface material layer. The composite structure layer has a cavity and includes a first structure layer, a second structure layer, and a connecting structure layer. The first structure layer includes at least one first conductive member, and the second structure layer includes at least one second conductive member. The cavity penetrates the first structure layer and the connecting structure layer to expose the second conductive member. The conductive structure at least penetrates the connecting structure layer and is electrically connected to the first conductive member and the second conductive member. The thermal interface material layer is disposed between the composite structure layer and the thermally conductive substrate, and the second structure layer is connected to the thermally conductive substrate through the thermal interface material layer.