An optical transmitter with a heat dissipation structure is provided. The heat dissipation structure comprises a substrate and an optical transmitter unit. The substrate comprises a base body, a heat dissipation well disposed on the base body, and a thermal conductive block inserted into and fixed to the heat dissipation well. The thermal conductive block has on one side thereof a heat guiding plane. The optical transmitter unit comprises a heat dissipating substrate directly disposed on the heat guiding plane, and a laser diode directly disposed on the heat dissipating substrate. The laser diode features an active region whose height is lowered to shorten a heat conduction path wherein heat is transferred from the active region through the heat dissipating substrate to the heat guiding plane. The heat already transferred to the heat guiding plane is transferred horizontally by the thermal conductive block to the base body which encloses the heat dissipation well.