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Hybrid aspen, a hybrid between the European aspen and North American trembling aspen (Populus tremula L. × P. tremuloides Michx.), is a promising tree species in northern Europe especially for plywood and biomass because of the fast-growing tree characteristics and short rotation period for final cutting. This research was carried out to analyze the tree diameter at breast height (DBH) and tree height of hybrid aspen plantations based on the repeatedly measured clonal trials in southern Finland. The experiments were established in four locations from 1997 to 1999 and measured 7-12 times by experiments. In each experiment, trees were planted with different initial planting density ranging from 400 trees per ha to 1600 trees per ha (5.0 m × 5.0 m spacing to 2.5 m × 2.5 m spacing). The results showed that the DBH distribution by stand density was the same in the beginning of the measurement, but it differed in all experiments over time. As the stand age increased, the DBH distribution was skewed to the right, which means the stand mean and median DBH were larger in the stands with 5.0 m × 5.0 m spacing than in the stands with 2.5 m × 2.5 m spacing. On the other hand, height distribution displayed the different growing rate characteristics. As the stand density increased, a larger proportion of trees’ heights were located in higher classes. The findings are considered to be useful for stand management of hybrid aspen.