Abstract: Abstract: Water transportation in plants has been an important issue in plant physiology and ecophysiology. Limited by experimental conditions, the flow patterns of water through xylem vessels, especially through the vessels with wall thickenings, is not easily perceived, and it is critical to understand the water transportation through plants. The aim of this study was to investigate the hydrodynamic properties and the detailed flow patterns occurring in xylem vessels with annular (helical) thickenings in order to obtain a functional interpretation of these structures. For this purpose, the flow of water through xylem vessels with wall thickenings was studied by adopting a computational fluid dynamics (CFD) approach here. For the computation approach, the Bernoulli mathematical model of the annular vessel was established based on the energy conservation law. According to the obtained mathematical model, the geometrical structures of vessel, such as the inner diameter of vessel, the distance between the thickenings, the height of thickenings, the width of thickenings and the inclination of thickenings, were the main factors that affected the vessel flow resistance. Numerical calculations based on shear stress transport (SST) k-ω turbulence model were implemented to simulate the flow in xylem vessels with various wall thickening structures. In the simulation experiments, we studied and analyzed various aspects, such as influences of inner diameter, distances among thickenings, heights and widths of thickenings, and inclinations of thickenings to the flow resistance coefficient. The results showed that the fluid resistances depended largely on vessel diameters, distances among the thickenings, and heights of thickenings. When other parameters were initialized, with the increase of vessel inner diameter, the average flow raised, while the pressure drop and flow resistance coefficient decreased, for instance, the flow resistance coefficients ranged from 21050.1 to 811.9. With the increase of the distance among thickenings, the average flow remained unchanged, while the pressure drop and flow resistance coefficient decreased, for instance, the flow resistance coefficients ranged from 10078.9 to 2369.9. With the increase of heights of thickenings, the average flow remained unchanged, while the pressure drop and flow resistance coefficient increased, for instance, the flow resistance coefficients ranged from 2032.6 to 20452.1. In contrast, the width and inclination of thickenings had little effect on the fluid resistance of vessel. It was noteworthy that the wall thickenings in small vessels contributed a large fraction of resistance to flow, whereas the wall thickenings in vessels with larger diameter contributed a relatively small fraction of resistance to flow. For example, the flow fraction of resistance generated by the thickenings was 57.0% in a vessel with an inner diameter of 16 μm when given the thickening height of 2.3 μm, while it was reduced to 27.2% in a vessel with an inner diameter of 50 μm. Under the same condition of structural parameters, the flow resistance of annular vessels was closed to that of helical vessels, and the flow resistance difference between annular vessel and helical vessels was less than 0.5% with the increase of the vessel diameter. The results above suggested that it was appropriate for the proposed mathematical model to consider the structures with complex geometries, and it was suitable for the CFD model based on SST k-ω turbulence model to simulate the flow through wall thickening structures in plant vessel, and to acquire the flow field parameters for which it was difficult to obtain by experiments. The proposed numerical simulation method provides a valid tool for further studies on the hydrodynamic characteristics of the plant vessels. Further studies on the properties of vessel structures are required in order to obtain detailed information about the interaction between wall thickenings and other functionally structures such as pits, and perforation plates.