Abstract: In order to understand the effect of root chemical components on its mechanical properties, root tensile tests were conducted on 473 roots with diameters of 0.75-7.65 mm from two conifers and three broadleaf, namely Pinus tabulaeformis Carr., Larix principis-rupprechtii Mayr., Betula platyphylla Suk., Quercus mongolicus Fisch. ex Ledeb. and Ulmus pumila Linn, which were very common species in North China. All tested roots were devided into 7 diameter classes to measure the cellulose content, lignin content, hemicellulose content and holocellulose content. The results showed that the root tensile force varied from 8 to 954 N and tensile strength was in the range of 6-53 MPa. There was significant difference in tensile force and tensile strength among roots of five species, with a clear following ranking order: Ulmus pumila Linn. has the largest tensile force and tensile strength, followed by Betula platyphylla Suk, Quercus mongolicus Fisch. ex Ledeb, Pinus tabulaeformis Carr, and Larix principis-rupprechtii Mayr. The tensile force increased with the increasing root diameter at the level of power function, and at the same time, the tensile strength decreased with the decrease in root diameter at the level of power function and inverse function. The range of root ultimate elongation was from 6.95% to 15.50% with significant difference among different tree species. The cellulose contents were from 20.09% to 37.67%, the lignin contents from 18.03% to 41.67%, the hemicellulose contents from 1.29% to 14.90%, the holocellulose contents from 26.20% to 52.09%, and the content ratio of lignin to cellulose was from 0.53 to 1.81. The root tensile force was positively correlated with cellulose content, hemicellulose content and holocellulose content, and was negatively correlated with lignin content and the content ratio of lignin to cellulose. The root tensile strength was negatively correlated with cellulose content, hemicellulose content and holocellulose content, but was positively correlated with lignin content and the content ratio of lignin to cellulose. For different tree species, the root chemical components effecting on the tensile force and tensile strength were different. Therefore, the size effect of root diameter on tensile strength can not be totally explained by the changes in root chemical components. Maybe the other inner factors of the root, such as its microstructure, also make the important contribution to it.