A power transmission apparatus for a vehicle includes: an input shaft receiving engine torque; a compound planetary gear set formed by first and second planetary gear sets and having first to fifth elements; a first shaft fixedly connected to the first element, fixedly connected to a motor/generator, and selectively connected to the input shaft; a second shaft fixedly connected to the second element and selectively connected to the input shaft; a third shaft fixedly connected to the third element and selectively connected to a housing; a fourth shaft fixedly connected to the fourth element, fifth shaft fixedly connecting the fifth element and an output gear; a sixth shaft selectively connected to the second shaft and the fourth shaft and selectively connected to the housing; engagement elements including a clutch and a brake; and an engagement unit selectively connecting the sixth shaft to the second shaft or the fourth shaft.