Abstract: The aim of this study was to determine the effects of Vetiveria zizanioides plants and their roots on soil properties, and define the role of different root diameter classes. The research was conducted in the Purple Soil Area of China. The experiment compared bare control plots with plots planted in V. zizanioides. Soil and root samples were collected regularly from 0-10 cm, 10-20 cm, 20-30 cm soil layers. Contents of soil organic matter, total and available nitrogen, phosphorus, and potassium were measured by conventional methods. Roots were divided into 10 diameter classes measured by a Root Analysis System (WinRHIZO, Pro. 2004c). It was found that: 1) V. zizanioides substantially enhanced the accumulation of soil organic matter, the longer the planted time the greater the effect. V. zizanioides also promoted the accumulation of soil total nitrogen, but the effect was weak compared with organic matter. Both available phosphorus and potassium were depleted in all V. zizanioides plots. 2) The levels of organic matter, available phosphorus, total potassium and available potassium showed topsoil accumulation in all V. zizanioides plots; i.e. over time their levels in the 0-10 cm soil layer increased significantly more than in the 10-20 cm and 20-30 cm soil layers (P<0.05). 3) The root diameter range (d) 0.00 mm<d≤2.00 mm was dominant in affecting soil organic matter. root length density (RLD), Root surface area density (RSAD), and root volume density (RVD) of all diameter classes in this range were significantly and positively correlated with soil organic matter content (P<0.01), with the highest correlation being the 0.00 mm<d≤0.50 mm diameter range and the next highest being the 1.50 mm<d≤2.00 mm range. 4) Root diameters of d=2.00 mm and d=4.50 mm were important classification divisions. When 0.00 mm<d≤2.00 mm, RLD, RSAD, and RVD of all diameter classes showed significant differences between all soil layers and were significantly and negatively correlated with soil layer depth. The maximum correlation coefficient occurred in roots of the 0.50 mm<d≤1.00 mm diameter range. When 2.00 mm<d≤4.50 mm, all root parameters of all diameter classes were uncorrelated with any measured soil parameter. 5) The maximum RLD, RSAD, and RVD occurred in the 0-10 cm soil layer, and RLD differed significantly among all soil layers in the same year or between 2011 and 2013 in the same soil layer.