Abstract: This paper discussed the optimum selection ages(xmax) of height, diameter and volume in 1-69 poplar x Simon poplar and 1-69 poplar x Nigra poplar F1 clones, using annual genetic gain(△Gt) and expected genetic gain(Gt) as the evaluation criteria, and the stability of their early selection result was tested. Age of early selection might be as ealy as 2 years after plantation(not including plantation age) in 1-69 poplar x Simon poplar Ft clones. Age of early selection was 3~4 years after plantation(including plantation age) in 1-69 poplar x Nigra poplar F, clones. Selection for these two F1 clones at these ages would represent that at rotation ages, and genetic gain of the selection would achieve the maximum value. Therefore, it was concluded that early selection would be effective.