Abstract: From the observation of 1eaf area index (LAI) of 7 Larix principis-rupprechtii plantation plots at 3 layers (tree canopy, shrubs and herbage) on the north slope in the vegetation period (May-Oct.) of 2009, the change of LAI along slope position, the vertical composition of LAI and its seasonal variation were studied at the semi-arid watershed of Diediegou, which locates at Liupan Mountains of Northwest China. The results showed: 1) The LAI of tree canopy decreased basically with rising slope position, from 1.52—2.95 at slope foot to 0.15—0.27 at slope top; the LAI of shrubs layer increased at first and then decreased with slope position, from 0.05—0.06 at slope foot to 0.73—1.01 at up-middle slope and 0.19—0.30 at slope top; while the LAI of herbage layer increased slowly, from 0.18—0.36 at slope foot to 0.19—0.75 at slope top. 2) The seasonal variation of LAI of tree canopy and herbage layers showed the same tendency, i.e. a one-peak curve of increase and then decrease with time. However, the LAI of tree canopy layer grew faster than that of herbage layer in the early vegetation period (May-Jun.) and slower than that of the herbage layer in the middle vegetation period (Jul.–Aug.), due to the integrated result of varying temperature and soil moisture as well as the root depth difference between trees and grasses. 3) With increasing tree canopy density, the LAI of each vegetation layer responded differently. The tree canopy LAI nearly linearly increased with canopy density; the LAI of shrubs layer increased at first and then decreased, with a maximum when the canopy density was about 0.5 and decreased to nearly zero when canopy density above 0.9; the LAI of herbage layer decreased gradually, from 0.74 to 0.35; the LAI of undergrowth (shrubs plus herbage) reached its maximum value and higher than tree canopy LAI when canopy density varied between 0.4—0.5, but decreased rapidly and lower than canopy LAI when canopy density above 0.6; the total LAI of all the 3 layers increased at first with rising canopy density, reaching its maximum and keeping relatively stable when canopy density was 0.6—0.8, and then slightly decreased with rising canopy density.