Power transmission system of hybrid electric vehicle

A power transmission system of a hybrid electric vehicle comprising:

• an input shaft receiving torque of an engine;
• an output shaft parallel with the input shaft;
• a first planetary gear set on the input shaft, and including a first rotation element connected to a first motor/generator, a second rotation element connected to the output shaft through an externally-meshed gear, and a third rotation element directly connected to the input shaft;
• a second planetary gear set along a center axis of the input shaft, and including a fourth rotation element directly connected to a second motor/generator and selectively connected to the third rotation element, a fifth rotation element connected to the output shaft through an externally-meshed gear, and a sixth rotation element selectively connected to a transmission housing;
• transfer gears forming the externally-meshed gears;
• and
• frictional elements selectively connecting two rotation elements among three rotation elements of the second planetary gear set, selectively connecting the third rotation element to the fourth rotation element, or selectively connecting the sixth rotation element to the transmission housing.