A power transmission apparatus of a hybrid electric vehicle, may include an engine and first and second motor/generator, the power transmission apparatus includes a planetary gear set including first, second, and third rotation elements, a first shaft fixedly connected to the first rotation element and a rotor of the first motor/generator, a second shaft fixedly connected to the second rotation element and the engine, a third shaft fixedly connected to the third rotation element, a fourth shaft externally meshed with the second shaft and the third shaft, a fifth shaft fixedly connected to a rotor of the second motor/generator, a sixth shaft externally meshed with the fourth shaft and the fifth shaft and transmitting rotational power of the engine and the first and second motor/generators to a final reduction gear of a differential gear and three gear trains externally engaged with the first to sixth shafts.