A junction box includes a cable connecting box and a cover covering the cable connecting box. The cable connecting box includes an insulative block, a plurality of contacting foils retained in the insulative block and a plurality of diodes. Each diode connects with two neighborly contacting foils. The contacting foils are insert-molded with the insulative block. Based on thermal radiation properties of the plastic material is superior to that of the metal material, as the contacting foils transfer the heat to the insulative block, the insulative block would deliver the heat to the cable connecting box, then the heat would be delivered to the cover. Such that, the heat dissipates outside quickly from the junction box.