A power transmission system of hybrid electric vehicle includes an input shaft, counter shaft, an output shaft, first motor/generator, second motor/generator, first planetary gear set, second planetary gear set to selectively externally gear-connect fourth rotation element to third rotation element through counter shaft while being directly connected to counter shaft and to directly connect fifth rotation element to output shaft and selectively connect sixth rotation element to transmission housing, first transfer gear disposed between second rotation element and output shaft, second transfer gear disposed between third rotation element and counter shaft, variable direct connection device to selectively connect two of three rotation elements of second planetary gear set, and variable connection device to selectively connect sixth rotation element to transmission housing or selectively external gear-connect third rotation element to fourth rotation element.