The effect of dye-doping in ambipolar light-emitting organic field-effect transistors (LE-OFETs) is investigated from the standpoint of the carrier mobilities and the electroluminescence (EL) characteristics under ambipolar operation. Dye doping of organic crystals permits not only tuning of the emission color but also significantly increases the efficiency of ambipolar LE-OFETs. A rather high external EL quantum efficiency (similar to 0.64%) of one order of magnitude higher than that of a pure p-distyrylbenzene (P3V2) single crystal is obtained by tetracene doping. The doping of tetracene molecules into a host P3V2 crystal has almost no effect on the electron mobility and the dominant carrier recombination process in the tetracene-doped P3V2 crystal involves direct carrier recombination on the tetracene molecules.