The leaf area index (LAI) is an important parameter for the evaluation of forest growth, stand structure, and numerous ecological services. It is necessary to understand and quantify the LAI variation with site conditions and its spatial scale effect on slopes. In this study, one representative southeast-facing slope covered by 33-years-old Larix principis-rupprechtii plantation was selected in the small watershed of Xiangshuihe within the semi-humid region of Liupan Mountains, northwest China. This slope has slope length of 480.6 m and a corresponding horizontal length of 398.2 m. A survey transect with a width of 30 m was set up on the whole slope, and sixteen stand plots were set up continuously within this survey transect, this means that the plots covered all different slope positions from slope top to slope foot. The canopy LAI in the middle of growing season of 2014 was measured and the spatial variation of LAI on the slope was analyzed. The results showed that there is a remarkable difference in canopy LAI among the different slope positions. The mean LAI on the whole slope was 3.11, with a variation range of 2.66-3.49, a difference of 0.83, and a coefficient of variation of 0.09. The variation of LAI along lowering slope positions showed a trend of firstly increase, reaching its maximum at the middle slope (at the horizontal slope length of 188.45 m), and thereafter a slight decrease. The canopy LAI showed a slope scale effect, i.e., the moving average of LAI (Y₁) increases gradually with the increasing horizontal slope length (X, m) from slope top, with an mean rate of 0.12 per 100 m. The corresponding relation to express this scale effect on the studied slope is Y₁=-2×10^-8X³+8×10^-6X²+5×10^-4X+2.6523(R²=0.99). The ratio of plot LAI to the whole slope average (Y₂) varies nonlinearly along the horizontal length of plots from slope top (X₁, m), with the relation of Y₂=-9×10^-8X³₁+2×10^-6X²₁+1×10^-3X₁+0.829(R²=0.78). This relation can be used to estimate the slope mean LAI from the LAI measured at certain slope position. The slope variation of LAI mentioned above is mainly caused by the difference of air temperature and soil moisture among the plots with different slope position and elevation.