Based on the mechanics of material, the bending stiffness and intensity of cylindrical bar and tube are analyzed. By comparing the cylindrical tube whose ratio of outside diameter to internal diameter is 0.7 with the cylindrical bar, it is concluded that when both of them have the same mass, the section stiffness of the cylindrical tube is three times that of the cylindrical bar; when both of them have the same external diameter, the mass of the cylindrical tube is only 1/2 that of the cylindrical bar, but the section stiffness of the cylindrical tube is 3/4 that of the cylindrical bar.By virtue of the elemental elastic-plastic theory, the yield stress of the liquid-filled cylindrical tube is investigated. Due to the incompressibility of liquid and the strain hardening effect of material, the yield stress of the liquid-filled tube is enlarged compared with the hollow tube, thus raising its bending intensity. Under the dynamic load, compared with the hollow tube, the impact resistance of the liquid-filled tube is also raised due to elastic recovery. Because the hydraulic pressures perpendicular to the inner surface are identical everywhere, the local stress concentration resulting from the ovalisation of the tube would be decreased, and the resistance to buckling would be improved.