Aims <p>Plant roots play a crucial role in soil stability and erosion prevention. Most studies currently focus on the macro-biomechanical properties of roots based on apparent diameter or stele size. However, these analyses cannot explain the factors affecting macro-biomechanical properties of roots from an endogenous perspective.</p> Methods <p>Tensile tests, scanning electron micrography (SEM), image-based strain measurement and compositional tests were conducted on roots of typical species (<i>Robinia pseudoacacia</i>, <i>Pinus tabuliformis</i>, <i>Vitex negundo</i>, <i>Syzygium aromaticum</i>) in the Loess Plateau to explore the influence of stele on “enhancing” and pores on “weakening” mechanical properties.</p> Results <p>Root breakages in tension can be categorized into simultaneous and successive brittle breakage, with most simultaneous brittle breakages occurring in fine roots and most successive brittle breakages occurring in coarse roots, respectively. The negative regression between tensile strength (<i>T</i>_<i>r</i>) and diameter (<i>D</i>_<i>r</i>) was attributed to the decrease in cellulose content. The positive regression between <i>T</i>_<i>r</i> and stele percentage was attributed to the dominant distribution of cellulose within the stele of root. Pores in plant root could weaken the macro-biomechanical properties, with trees generally having higher porosity than shrubs in this research species. The non-uniformity coefficient (UC) of pores reflected their distribution form. The fine roots, with higher UC, showed more random pore distribution, more scattered macro-biomechanical properties than coarse roots.</p> Conclusions <p>Our results explained the intrinsic characteristics that influence the macro-biomechanical properties along root diameters. This finding provides valuable insights for understanding the mechanical properties of plant roots and providing soil reinforcement theoretical basis.</p>