Aims Our objectives were to: 1) compare the advantages and disadvantages of the root order and diameter classes in the description of root morphology characteristics and 2) explore the inherent connection between them. Methods We excavated 2-year-old Pinus tabulaeformis seedling roots in Zhouzhi County, Shaanxi Province, China. Individual roots were dissected according to the branching order, starting from the distal end of the root system that was numbered as the first order and then increasing sequentially with each branch from the first order roots to higher order ones. Following dissection, fine root samples were scanned by the Win-RHIZO system to analyze root length, specific root length, specific root surface area, diameter and tips. Fine root samples were then oven-dried at 65 °C to a constant weight to determine weight. Important findings Six root orders were included, with diameters that ranged from 0.169 to 3.877 mm. The summed length and surface area of the first three orders accounted for 78.77% and 62.72% of the totals, respectively. The specific root length and specific root area of the first three orders are 1.3–3.0 times and 1.0–1.5 times that of as the other orders. Based on diameter classes of ≤0.5, 0.5–1.0, 1.0–1.5 and 1.5–2.0 mm, most root diameters of P. tabulaeformis seedling were <1.5 mm. The summed length and surface area of this diameter range accounted for 93.76% and 84.35% of the totals, respectively. The specific root length and specific root area of roots <1.5 mm in diameter were 3.0–7.0 times and 1.5–3.0 times as large as roots >1.5 mm. For the exponential relationship between root orders and diameters and based on diameters’ maximum ratio reflecting root orders, a new diameter classification was proposed: ≤0.4, 0.4–0.8, 0.8–1.3 and 1.3–2.0 mm. As a result, the above intervals might represent the I, II, III, IV and V root orders in number of root tip of 93.22%, 86.37%, 75.96%, 70.47% and 76.67%, root length of 89.34%–70.83%, root area of 86.01%–76.12% and root volume of 87.73%–76.12%. Therefore, we identified a relationship between root diameter classification and root order, and it is possible to reflect root orders through logical division of the root diameter classes.