This study was carried out to find the suitable root-ball size for the planting construction of landscape trees. Surveyed trees for this study were Cercidiphyllum japonicum and Prunus yedoensis, and the root-ball size was classified into 3D, 4D, 5D, and 6D(D meaning the diameter at the base of a tree). Visual ratings on a scale of 1 to 9 were used as a means for measuring the regrowth strength after planting tested trees. Test trees with 3, 4, 5 and 6D root-ball size were planted on March. 16, 2002 and visual ratings were measured up to April. 30, 2003. Cercidiphyllum japonicum and Prunus yedoensis started showing a difference in regrowth by each rootball size a month after planting. The regrowth quality of Cercidiphyllum japonicum was low at 3D, medium at 4∼5D and high at 6D root-ball size, while the regrowth quality of Prunus yedoensis was low at 3∼4D and medium-high at 5∼6D root-ball size. According to the above results, a root-ball size for good regrowth quality was a little bit different between Cercidiphyllum japonicum and Prunus yedoensis. 4D of Cercidiphyllum japonicum and 5D of Prunus yedoensis could be guidelines for root-ball diameters at the base of trees when planting. However, we concluded that 4D∼5D root-ball is the optimum guideline for regrowth when adapting this guideline to all landscape trees with 8∼l0cm diameter at the base.