The heat dissipation structure of a horizontal optical-communication sub-assembly is provided, which includes a T-shaped header, a circuit board and a heat-dissipating support insert. The T-shaped header includes a base and a tongue. The tongue is disposed on one side of the base and is perpendicular to the base. The base includes a first through hole and a second through hole. The first through hole is above the tongue and the second through hole is below the tongue and opposite to the first through hole. One end of the circuit board penetrates through the first through hole and disposed on the tongue. The heat-dissipating support insert includes a supporting block and an extension portion. The extension portion is disposed on one side of the supporting block and penetrates through the second through hole to extend to the bottom of the tongue.