Abstract: Ability of the sweet potatoes Quanshu No.9,to provide high yield and its mechanism are briefly described.Quanshu No.9 was bred by Quanzhou Institute of Agricultural Science in 2003 from the female parent Quanshu 268 by open-pollinating.The new variety was examined and approved by the Crop Variety Approval Committee of Fujian in 2009.Quanshu No.9 had an annual yield of fresh roots ranging from 27 859.5 to 49 750.5 kgm--2,which was 89.81%-127.05% of the control(Jinshang 57 or Nangshu 88).Its annual yield of dry roots was between 8 500.0 and 14 929.5 kgm-2,which was 104.20%-152.30% of the control.The annual starch yield was between 5 620.5 and 9 850.5 kgm-2,which was 107.91%-155.36% of the control.The high yield was theorized to be contributed by the fast and stable root growth and the slow deterioration of stems and leaves during mid-to late-period.This mechanism of high leaf area,net assimilation rate and economic coefficient resulted in the high yield.With its fine quality,multi-disease-resistance and high starch content,Quanshu No.9 was fit for either fresh-eating or starch-processing.Its acreage in the province during 2009 was about 3 000 hm2,and appeared to be particularly promising in the south and the middle and lower reaches of Yangtze River.